Another youthful star makes the news today, the eight million year old HR 4796A in Centaurus, some 220 light years from Earth. As we saw yesterday, we have much to learn about how planets form around young stars. This one hasn’t yielded a planet, but its dust disk, discovered in 1991, seems to derive from a planetary system in formation, the evident product of collisions between small bodies called planetesimals.
The latest work on HR 4796A draws on observations made by the Near-Infrared Multi-Object Spectrometer aboard the Hubble Space Telescope. The spectra that John Debes and Alycia Weinberger (Carnegie Institution, Washington) studied in visible and infrared light scattered by the star’s disk look red and imply the existence of the large organic carbon molecules called tholins. These are organic aerosols, complex molecules that, on Titan at any rate, remain suspended in the atmosphere and may contain chemical precursors to life.
Image: Red and near infrared wavelengths from the dust disk surrounding the star HR 4796A (masked in false-color image to make fainter disk visible) suggest the presence of complex organic molecules. The inner “hole” of the ring-shaped disk is big enough to fit our entire solar system and may have been swept clean of dust by orbiting planets. Credit: John Debes.
How sure are we of the tholin identification? The authors are understandably cautious, saying:
Longer wavelength scattered light observations will further constrain the grain models we have used, particularly around 3.8-4µm where a large absorption feature is seen for different grain sizes of tholins. This would help to directly confirm whether Titan tholins are an adequate proxy for the material in orbit around HR 4796A. Additionally, measuring the optical properties of organic materials in meteorites and from samples of the Stardust mission will provide further tests of our model grains.
If ‘tholins’ ring a bell, it may be because we’ve looked at them before as a factor on Titan, just one place in the Solar System they have been detected. But until now (and assuming these findings are confirmed) tholins have not been found outside the vicinity of our Sun. That makes this result interesting in itself, but even more so when placed in context. HR 4796A is twice as massive as the Sun and twenty times more luminous. Learning how planets form and looking for possible ways for life to evolve in systems sharply different from our own may teach us much about the broader mechanisms at work around other stars.
The paper is Debes, Weinberger et al., “Complex Organic Materials in the Circumstellar Disk of HR 4796A,” to be published in Astrophysical Journal Letters and available in preprint form online.
Its odd how we can see tholins and other complex carbon molecules forming so early after a star’s birth and we can see these molecules spread across our own system’s planets such as Titan etc, but we still cannot figure out how these molecules turn into the initial forms of life.
Or am i the only one thinking this is mysterious? I dont mean any telelogical implications just that we must be missing something because if these raw materials for life are so abundant then where are the molecules which represent the next step of chemical evolution after the tholin/PAHs stage?
An interesting question, and one I’d like to know more about as well. Do we have any chemists in the house with an astrobiological bent?
Interesting these are being found around an A-type star, which puts out a lot of ultraviolet… I wonder how this result affects conclusions about the concept of a “UV habitable zone”. Or maybe the system is young enough that there hasn’t been time to do significant damage to the organics.
I suspect the latter, andy, but this idea of a UV habitable zone is an interesting one that clearly has a long way to evolve. I’m aware of one useful paper on this — Buccino et al., “Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones,” which I think wound up in Icarus:
http://arxiv.org/abs/astro-ph/0512291
and would appreciate any other citations readers have about the topic.
Hi All
The step from assorted amino acids to nucleotides is quite hard and requires a pretty restrictive set of conditions. The nucleobase adenine has been found in an interstellar cloud, but a few more bits need to be added to make a nucleotide, then the nucleotides have to be strung together. It’s all chemistry that can be done in a lab, but hard to replicate in open space. All formation scenarios require the components to be concentrated, either via evaporation or mineral confinement, plus some sort of energy flow through the system – even harder to do in deep space. Aqueous chemistry may not be essential for Life, but it is essential for the kind of chemistry we currently recognize as Life, and so any location without abundant liquid water isn’t going to progress much beyond what we see.
http://www.seti.org/nai/research_earlyEarth.php
I was googling for tholins +UV screen because i remember hearing that tholins also act as a protective agent against UV. I think this makes it quite facinating as perhaps both a raw material for life and the protection of more complex molecules or life itself from harmful UV light.
This star and protoplanetary disk discovery is very fascinating. It makes me wonder how many red dwarfs might their be within a few hundred light year radius of Earth that we have not discovered yet. Red dwarfs have long intrigued me as stars that could support life on closely orbiting planets and which might have indiginous ETI that have been evolving for over 13 billion years. Some of these red dwarfs might make excellent habitable zones for future humanity especially since their projected lifetimes with stable power output range as high as 10 EXP 15 years. I wonder how cool it would be to look out the window of a spaceship in orbit around one such star and notice its bright red disk. I guess the same could be said about a white star or a blue supergiant althought because of the greater light flux densities per unit of spherical angle subtended by these later examples, looking directly at such hot stars, even for a casual quick glance might be highly damaging to the human eye.
Jim, the number of red dwarfs within, say, five hundred light years may well grow as we improve our telescopes, but I’m also curious how many brown dwarfs may be out there within a reasonable distance. Could there be a brown dwarf closer than the Centauri trio? I don’t think we can rule that scenario out, but I’ll bet within a decade or so we can speak more definitively on the subject.
Hi Paul;
If there happened to be one or more brown dwarfs within a light year radius of Earth, perhaps even within a fraction of a light year radius, these stars might make excellent target destinations for manned mildly relativistic space craft such as fusion rocket powered craft in order to test flight hardware, ways of dealing with interstellar debris, collision avoidance remote sensing technologies, and slow down non-rocketed assisted technologies such as magnetic breaking etc. This would be a good way to practice mildly to moderately relativistic manned missions to any of the nearest stars some of which perhaps might have inhabited planets in orbit around them. Also, it would be interesting to see if any planets might have coalesced around any such brown dwarfs. Such planets could be useful for the raw materials they might provide for deep solar system/Oort cloud colonies and research outposts in the comming centuries. Just from the point of view of novelty’s sake, I think discovery of such nearby brown dwarfs would be fascinating.
Thanks;
Jim
With regard to red dwarfs, I do not think that long lifespan per se is any better guarantee for life, but rather the favorable conditions, biochemical ‘driving mechanisms’.
E.g., life arose on earth already within a couple of hundred million years.
But yes, if and once life is established, it could last almost ‘for ever’ on a planet near a red dwarf.
There are other constraints on planetary habitability other than stellar evolution: the planet’s interior cooling down is one – this could lead to the geomagnetic field disappearing. Another could be increasing continental area (in general, continents don’t get subducted), which provides a larger surface area over which chemical weathering can remove atmospheric CO2, perhaps leading to planetary death by freezing, particularly if the volcanic activity replenishing the CO2 is also falling off. I’ve seen models which suggest the maximum habitable lifetime of an Earth-mass terrestrial planet is about 6-7 billion years, which is far less than the main sequence timescale of a red dwarf.
Photophoretic Structuring of Circumstellar Dust Disks
Authors: Taku Takeuch, Oliver Krauss
(Submitted on 7 Jan 2008)
Abstract: We study dust accumulation by photophoresis in optically thin gas disks. Using formulae of the photophoretic force that are applicable for the free molecular regime and for the slip-flow regime, we calculate dust accumulation distances as a function of the particle size. It is found that photophoresis pushes particles (smaller than 10 cm) outward. For a Sun-like star, these particles are transported to 0.1-100 AU, depending on the particle size, and forms an inner disk. Radiation pressure pushes out small particles (< 1 mm) further and forms an extended outer disk. Consequently, an inner hole opens inside ~0.1 AU. The radius of the inner hole is determined by the condition that the mean free path of the gas molecules equals the maximum size of the particles that photophoresis effectively works on (100 micron – 10 cm, depending on the dust property). The dust disk structure formed by photophoresis can be distinguished from the structure of gas-free dust disk models, because the particle sizes of the outer disks are larger, and the inner hole radius depends on the gas density.
Comments: 15 pages, 9 figures, Accepted by ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.0907v1 [astro-ph]
Submission history
From: Taku Takeuchi [view email]
[v1] Mon, 7 Jan 2008 06:28:48 GMT (128kb)
http://arxiv.org/abs/0801.0907
The Evolutionary State of Anemic Circumstellar Disks and the Primordial-to-Debris Disk Transition
Authors: Thayne Currie (Harvard-Smithsonian Center for Astrophysics; Department of Physics and Astronomy, University of California-Los Angeles)
(Submitted on 7 Jan 2008)
Abstract: We investigate the evolution of $\sim$ 3 Myr-old MIPS-detected circumstellar disks in IC 348 that may be in an intermediate stage between primordial, optically-thick disks of gas/dust and debris disks characteristic of the final stages of planet formation. We demonstrate that these \textit{anemic} disks are not a homogenous class of objects corresponding to a unique evolutionary state. Rather, such disks around early (B/A) spectral type stars are most likely warm, terrestrial zone debris disks; MIPS-detected anemic disks around later (M) stars are likely \textit{evolved primordial disks} such as transition disks in their mid-IR colors, accretion signatures, and disk luminosities. Anemic disks surrounding G and K stars contain both populations. The difference in evolutionary states between anemic disks surrounding early type vs. late-type stars is consistent with a mass-dependent evolution of circumstellar disks from the primordial disk phase through the debris disk phase. Specifically, disks characteristically get to the debris disk phase faster for early-type stars; debris disks dominate the disk population of early-type stars by $\sim$ 3–5 Myr. Debris disks take $\sim$ 13–30 Myr to dominate the disk population around late-type stars.
Comments: 34 pages, 11 figures; submitted to MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.1116v1 [astro-ph]
Submission history
From: Thayne Currie [view email]
[v1] Mon, 7 Jan 2008 21:19:14 GMT (115kb)
http://arxiv.org/abs/0801.1116
Submillimeter Structure of the Disk of the Butterfly Star
Authors: S. Wolf, A. Schegerer, H. Beuther, D.L. Padgett, K.R. Stapelfeldt
(Submitted on 9 Jan 2008)
Abstract: We present a spatially resolved 894 micron map of the circumstellar disk of the Butterfly star in Taurus (IRAS 04302+2247), obtained with the Submillimeter Array (SMA). The predicted and observed radial brightness profile agree well in the outer disk region, but differ in the inner region with an outer radius of ~80-120 AU. In particular, we find a local minimum of the radial brightness distribution at the center, which can be explained by an increasing density / optical depth combined with the decreasing vertical extent of the disk towards the center. Our finding indicates that young circumstellar disks can be optically thick at wavelengths as long as 894 micron. While earlier modeling lead to general conclusions about the global disk structure and, most importantly, evidence for grain growth in the disk (Wolf, Padgett, & Stapelfeldt 2003), the presented SMA observations provide more detailed constraints for the disk structure and dust grain properties in the inner, potentially planet-forming region (inside ~80-120 AU) vs. the outer disk region (~120-300 AU).
Comments: 6 pages, emulatepaj, Accepted to ApJL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.1422v1 [astro-ph]
Submission history
From: Sebastian Wolf [view email]
http://arxiv.org/abs/0801.1422
Probing the centre of the large circumstellar disc in M17
Authors: M. Nielbock, R. Chini, V. H. Hoffmeister, D. E. A. Nürnberger, C. M. Scheyda, J. Steinacker
(Submitted on 10 Jan 2008)
Abstract: We investigated the nature of the hitherto unresolved elliptical infrared emission in the centre of the ~20000 AU circumstellar disc silhouette in M17. We combined high-resolution JHKLM band imaging carried out with NAOS/CONICA at the VLT with H2 and [Fe II] narrow band imaging using SOFI at the NTT. The analysis is supported by Spitzer/GLIMPSE archival data and by already published SINFONI/VLT Integral Field Spectroscopy data.
For the first time, we resolve the elongated central infrared emission into a point-source and a jet-like feature that extends to the northeast in an opposite direction to a recently discovered collimated H2 jet. We interpret the point-like emission as to originate from an accreting intermediate to high-mass protostar. In addition, our images reveal a weak and curved southwestern lobe whose morphology resembles that of the previously detected northeastern one. Both are located at a distance of 1500 AU from the disc centre. Such a jet is strongly suggested by H2 emission emerging from the disc centre toward the southwest along an axis that is almost perpendicular to the disc plane. The protostar is embedded in circumstellar material producing a visual extinction of Av greater than 60. The observed Ks band magnitude is equivalent to a main-sequence star having a spectral type of at least B8 that corresponds to a stellar mass of ~2.8 Msun. Altogether, we suggest that the large M17 disc silhouette is associated with an intermediate to high-mass protostar that accretes material from the disc and expels part of it through a symmetric bipolar jet or outflow.
Comments: 7 pages, 3 figures, submitted to MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.1578v1 [astro-ph]
Submission history
From: Markus Nielbock [view email]
[v1] Thu, 10 Jan 2008 11:29:20 GMT (497kb)
http://arxiv.org/abs/0801.1578
@andy Says:
January 7th, 2008 at 9:21
“There are other constraints on planetary habitability other than stellar evolution: the planet’s interior cooling down is one (…). Another could be (…) planetary death by freezing, particularly if the volcanic activity replenishing the CO2 is also falling off. I’ve seen models which suggest the maximum habitable lifetime of an Earth-mass terrestrial planet is about 6-7 billion years, which is far less than the main sequence timescale of a red dwarf.
”
That’s a very sobering thought, that a planet may have an intrinsic maximum lifespan, as a living planet, that depends not just on the lifespan of its star, but also on the size (mass) and geology of the planet itself.
If this appears to hold true, this should be taken into consideration when prioritizing future searches for biosignatures etc. and also when estimating the number of habitable planets: not only is minimum age of interest, but also maximum age! I have never seen this mentioned in any article about earthlike exoplanets, searches for biosignatures, SETI, etc.
…
Apart from the already well-known fact that the habitable zone of a (solar type) star moves outward over time, thus also limiting the lifespan of a habitable planet to (considerably) less than the total lifespan of the star. In the case of our sun, the earth will leave the habitable zone on the inzide (i.e. becoming too hot) in about 0.5 – 1 gy.
Putting it together it seems that the ideal (i.e. long-lasting) planet for biological life would be a relatively heavy terrestrial planet orbiting a relatively cool sunlike star (G3-G9, K0-K?).
Ronald: turns out there’s another reason why cooler G stars might be good – tides. Around an M dwarf the solar tides are so strong that a habitable planet’s rotation gets synchronised, around the Sun the solar tides are too weak to keep the Earth’s axis stable. Somewhere between the Sun (G2V) and the M dwarfs, the solar tides in the habitable zone are going to be comparable to the lunar tides on Earth – strong enough to stabilise the axis, but weak enough so that the planet doesn’t get synchronised if it lacks a large moon.
@andy:
andy, thanks !
Always thought that our sun, though acceptable, wasn’t the optimal star ;-)
Indeed, the slightly cooler G stars (G3 – G9, maybe K0) might be the optimal stars: lifespan, stability, light spectrum (less aggressive radiation, more photosynthetic light), and now also tides.
And they are (slightly) more abundant than the hotter stars as well.
Some good examples of this in out galactic neighborhood are: 61 Virginis (G5), Alpha Mensae (G5), Gliese 442A (G5, wide binary). 82 Eridani is also G5, but has rather low metallicity (?).
Resonance Trapping in Protoplanetary Disks
Authors: Aaron T. Lee, Edward W. Thommes, Frederic A. Rasio
(Submitted on 12 Jan 2008)
Abstract: Mean-motion resonances (MMRs) are likely to play an important role both during and after the lifetime of the protostellar gas disk. We study the dynamical evolution and stability of planetary systems containing two initially circular giant planets near 2:1 resonance and closer. We find that in addition to the 2:1, planets can capture into the 5:3 and 3:2. We use direct numerical integrations of approximately 1000 systems in which the planets are placed in one of these resonances and allowed to evolve for up to approximately 10^7 yr. We find that the final eccentricity distribution of those systems which ultimately become unstable gives a good fit to that of the observed planetary systems. Next, we integrate approximately 500 two-planet systems in which the outer planet is driven to migrate inward, resonantly capturing the inner; the systems are evolved until either instability sets in, or the planets reach the star. We find that although the 5:3 resonance rapidly becomes unstable under migration, the 2:1 and 3:2 are very stable.
Thus the lack of observed exoplanets in resonances closer than 2:1, if it continues to hold up, may be a primordial signature of the planet formation process.
Comments: 9 pages, 7 color figures. Submitted to the Astrophysical Journal
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.1926v1 [astro-ph]
Submission history
From: Aaron Lee [view email]
[v1] Sat, 12 Jan 2008 23:44:59 GMT (190kb)
http://arxiv.org/abs/0801.1926
Eccentricity growth of planetesimals in a self-gravitating protoplanetary disc
Authors: M. Britsch, C. J. Clarke, G. Lodato
(Submitted on 14 Jan 2008)
Abstract: We investigate the orbital evolution of planetesimals in a self-gravitating circumstellar disc in the size regime ($\sim 1-5000$ km) where the planetesimals behave approximately as test particles in the disc’s non-axisymmetric potential. We find that the particles respond to the stochastic, regenerative spiral features in the disc by executing large random excursions (up to a factor of two in radius in $\sim 1000$ years), although typical random orbital velocities are of order one tenth of the Keplerian speed. The limited time frame and small number of planetesimals modeled does not permit us to discern any {\it net} direction of planetesimal migration.
Our chief conclusion is that the high eccentricities ($\sim 0.1$) induced by interaction with spiral features in the disc is likely to be highly unfavourable to the collisional growth of planetesimals in this size range while the disc is in the self-gravitating regi me. Thus {\it if}, as recently argued by Rice et al 2004, 2006, the production of planetesimals gets under way when the disc is in the self-gravitating regime (either at smaller planetesimal size scales, where gas drag is important, or via gravitational fragmentation of the solid component), then the planetesimals thus produced would not be able to grow collisionally until the disc ceased to be self-gravitating. It is unclear, however,given the large amplitude excursions undergone by planetesimals in the self-gravitating disc, whether they would be retained in the disc throughout this period, or whether they would instead be lost to the central star.
Comments: To appear in MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.2108v1 [astro-ph]
Submission history
From: Cathie Clarke [view email]
[v1] Mon, 14 Jan 2008 16:19:13 GMT (259kb)
http://arxiv.org/abs/0801.2108
Flight of the Moth
http://www.astrobio.net/news/article2587.html
A dusty disk that resembles the shape of a moth
is providing new information about how planetary
systems form and evolve.
Magnetorotational instability in protoplanetary discs: The effect of dust grains
Authors: Raquel Salmeron (1 and 2), Mark Wardle (3) ((1) Planetary Science Institute, Research School of Astronomy & Astrophysics and Research School of Earth Sciences, The Australian National University, (2) Department of Astronomy & Astrophysics, The University of Chicago, (3) Physics Department, Macquarie University)
(Submitted on 16 Jan 2008)
Abstract: We investigate the linear growth and vertical structure of the MRI in protoplanetary discs when dust grains are well mixed with the gas over the entire disc thickness. All the grains have the same radius (a = 0.1, 1 or 3 micron) and constitute 1 % of the total mass of the gas. Solutions are obtained at R = 5 and 10 AU for a minimum-mass solar nebula model and different choices of the initially vertical magnetic field strength (B), configuration of the diffusivity tensor and grain sizes. We find that when no grains are present, or they are greater than 1 micron, the midplane remains magnetically coupled for B up to a few gauss at both radii. In contrast, when a population of small grains (a = 0.1 micron) is present, the disc is magnetically inactive for z/H less than 2 and only B less than 50 mG couple to the fluid. At 5 AU, Ohmic diffusion dominates for z/H less than 1 when B less than a few mG, irrespective of the properties of the grain population.
Conversely, at 10 AU this diffusion term is unimportant in all the scenarios studied here. For z/H greater than 5, ambipolar diffusion is severe and prevents the field from coupling to the gas for all B. Hall diffusion is dominant for a wide range of field strengths at both radii when dust grains are present. The growth rate, wavenumber and range of magnetic field strengths for which MRI-unstable modes exist are all drastically diminished when dust grains are present, particularly when they are small (a ~ 0.1 micron). We conclude that in protoplanetary discs, the magnetic field is able to couple to the gas and shear over a wide range of fluid conditions even when small dust grains are well mixed with the gas. Despite the low magnetic coupling, MRI modes grow for an extended range of magnetic field strengths and Hall diffusion largely determines the properties of the perturbations in the inner regions of the disc (abridged).
Comments: 17 pages, 11 figures. Submitted to MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.2428v1 [astro-ph]
Submission history
From: Raquel Salmeron [view email]
[v1] Wed, 16 Jan 2008 04:18:30 GMT (797kb,D)
http://arxiv.org/abs/0801.2428
On the observability of resonant structures in planetesimal disks due to planetary migration
Authors: Rémy Reche (LAOG), Hervé Beust (LAOG), J. C. Augereau (LAOG), Olivier Absil (LAOG)
(Submitted on 17 Jan 2008)
Abstract: We present a thorough study of the impact of a migrating planet on a planetesimal disk, by exploring a broad range of masses and eccentricities for the planet. We discuss the sensitivity of the structures generated in debris disks to the basic planet parameters. We perform many N-body numerical simulations, using the symplectic integrator SWIFT, taking into account the gravitational influence of the star and the planet on massless test particles. A constant migration rate is assumed for the planet. The effect of planetary migration on the trapping of particles in mean motion resonances is found to be very sensitive to the initial eccentricity of the planet and of the planetesimals.
A planetary eccentricity as low as 0.05 is enough to smear out all the resonant structures, except for the most massive planets. The planetesimals also initially have to be on orbits with a mean eccentricity of less than than 0.1 in order to keep the resonant clumps visible. This numerical work extends previous analytical studies and provides a collection of disk images that may help in interpreting the observations of structures in debris disks. Overall, it shows that stringent conditions must be fulfilled to obtain observable resonant structures in debris disks. Theoretical models of the origin of planetary migration will therefore have to explain how planetary systems remain in a suitable configuration to reproduce the observed structures.
Comments: 16 pages, 13 figures. Accepted for publication in A&A
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.2691v1 [astro-ph]
Submission history
From: Remy Reche [view email] [via CCSD proxy]
[v1] Thu, 17 Jan 2008 14:57:01 GMT (2184kb)
http://arxiv.org/abs/0801.2691
Accretion properties of T Tauri stars in sigma Ori
Authors: T. Gatti, A. Natta, S. Randich, L. Testi, G. Sacco
(Submitted on 18 Jan 2008)
Abstract: Accretion disks around young stars evolve in time with time scales of few million years. We present here a study of the accretion properties of a sample of 35 stars in the ~3 million year old star-forming region sigma Ori. Of these, 31 are objects with evidence of disks, based on their IR excess emission. We use near-IR hydrogen recombination lines (Pa_gamma) to measure their mass accretion rate.
We find that the accretion rates are significantly lower in sigma Ori than in younger regions, such as rho-Oph, consistently with viscous disk evolution. The He I 1.083 micron line is detected (either in absorption or in emission) in 72% of the stars with disks, providing evidence of accretion-powered activity also in very low accretors, where other accretion indicators dissapear.
Comments: Astronomy and Astrophysics, accepted
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.2886v1 [astro-ph]
Submission history
From: Antonella Natta [view email]
[v1] Fri, 18 Jan 2008 13:51:42 GMT (78kb)
http://arxiv.org/abs/0801.2886
Kinematics of solid particles in a turbulent protoplanetary disc
Authors: Augusto Carballido (JPL), James M. Stone (Princeton), Neal J. Turner (JPL)
(Submitted on 23 Jan 2008)
Abstract: We perform numerical simulations of solid particle motion in a shearing box model of a protoplanetary disc. The accretion flow is turbulent due to the action of the magnetorotational instability. Aerodynamic drag on the particles is modelled using the Epstein law with the gas velocity interpolated to the particle position. The effect of the magnetohydrodynamic turbulence on particle velocity dispersions is quantified for solids of different stopping times t_s, or equivalently, different sizes. The anisotropy of the turbulence is reflected upon the dispersions of the particle velocity components, with the radial component larger than both the azimuthal and vertical components for particles larger than ~ 10 cm (assuming minimum-mass solar nebula conditions at 5 AU). The dispersion of the particle velocity magnitude, as well as that of the radial and azimuthal components, as functions of stopping time, agree with previous analytical results for isotropic turbulence. The relative speed between pairs of particles with the same value of t_s decays faster with decreasing separation than in the case of solids with different stopping time. Correlations in the particle number density introduce a non-uniform spatial distribution of solids in the 10 to 100 cm size range. Any clump of particles is disrupted by the turbulence in less than one tenth on an orbital period, and the maximally concentrated clumps are stable against self-gravitational collapse.
Comments: 11 pages, 9 figures. Accepted for publication in MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.3646v1 [astro-ph]
Submission history
From: Augusto Carballido [view email]
[v1] Wed, 23 Jan 2008 19:16:11 GMT (94kb)
http://arxiv.org/abs/0801.3646
Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations
Authors: A. Crapsi (1,2), E. F. van Dishoeck (1), M. R. Hogerheijde (1), K. M. Pontoppidan (3), C.P. Dullemond (4) ((1) Sterrewacht Leiden (2) Observatorio Astronomico Nacional (3) GPS (4) Max-Plank-Institut fur Astronomie)
(Submitted on 27 Jan 2008)
Abstract: (Abridged) Classification schemes for YSOs are based on evaluating the degree of dissipation of the surrounding envelope, whose main effects are the extinction of the optical radiation from the central YSO and re-emission in the far-infrared. Since extinction is a property of column density along the line of sight, the presence of a protoplanetary disk may lead to a misclassification when the system is viewed edge-on.
We performed radiative transfer calculations, using the axysimmetric 3D radiative transfer codes RADMC and RADICAL, to show the effects of different geometries on the main indicators of YSO evolutionary stage, like the slope of the flux between 2 and 24mum, the bolometric temperature and the optical depth of silicates and ices.
We show that for systems viewed at intermediate angles the ‘classical’ indicators of evolution accurately trace the envelope column density, and they all agree with each other. On the other hand, edge-on system are misclassified for inclinations larger than ~65deg. In particular, silicate emission, typical of pre-main sequence stars with disks, turns into absorption when the disk column density reaches 1e22cm-2, corresponding e.g. to a 5e-3 Msun flaring disk viewed at 64deg. A similar effect is noticed in all the other classification indicators studied alpha, Tbol, and the H2O and CO2 ices absorption strengths. This misclassification has a big impact on the nature of the flat-spectrum sources (alpha ~0), whose number can be explained by simple geometrical arguments without invoking evolution. A reliable classification scheme using a minimal number of observations is constituted by observations of the mm-flux with both a single dish and an interferometer.
Comments: 14 pages, 6 figures. Accepted by Astronomy and Astrophysics
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.4139v1 [astro-ph]
Submission history
From: Antonio Crapsi [view email]
[v1] Sun, 27 Jan 2008 16:04:59 GMT (248kb)
http://arxiv.org/abs/0801.4139
Planet Shadows in Protoplanetary Disks. I: Temperature Perturbations
Authors: H. Jang-Condell
(Submitted on 29 Jan 2008)
Abstract: Planets embedded in optically thick passive accretion disks are expected to produce perturbations in the density and temperature structure of the disk. We calculate the magnitudes of these perturbations for a range of planet masses and distances. The model predicts the formation of a shadow at the position of the planet paired with a brightening just beyond the shadow. We improve on previous work on the subject by self-consistently calculating the temperature and density structures under the assumption of hydrostatic equilibrium and taking the full three-dimensional shape of the disk into account rather than assuming a plane-parallel disk. While the excursion in temperatures is less than in previous models, the spatial size of the perturbation is larger.
We demonstrate that a self-consistent calculation of the density and temperature structure of the disk has a large effect on the disk model. In addition, the temperature structure in the disk is highly sensitive to the angle of incidence of stellar irradition at the surface, so accurately calculating the shape of the disk surface is crucial for modeling the thermal structure of the disk.
Comments: 14 pages, 14 figures. To appear in ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.4561v1 [astro-ph]
Submission history
From: Hannah Jang-Condell [view email]
[v1] Tue, 29 Jan 2008 21:09:48 GMT (997kb)
http://arxiv.org/abs/0801.4561
The Evolution of Primordial Disks
Authors: Lucas A. Cieza
(Submitted on 29 Jan 2008)
Abstract: Circumstellar disks are an integral part of the star formation process and the sites where planets are formed. Understanding the physical processes that drive their evolution, as disks evolve from optically thick to optically thin, is crucial for our understanding of planet formation. Disks evolve through various processes including accretion onto the star, dust settling and coagulation, dynamical interactions with forming planets, and photo-evaporation. However, the relative importance and timescales of these processes are still poorly understood.
In this review, I summarize current models of the different processes that control the evolution of primordial circumstellar disks around low-mass stars. I also discuss recent observational developments on circumstellar disk evolution with a focus on new Spitzer results on transition objects.
Comments: 15 pages and 5 figures, invited review talk, to appear in the ASP conference proceedings of the “Frank N. Bash Symposium 2007: New Horizons in Astronomy”, editors: A. Frebel, J. Maund, J. Shen, M. Siegel
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.4572v1 [astro-ph]
Submission history
From: Lucas Cieza [view email]
[v1] Tue, 29 Jan 2008 21:55:35 GMT (675kb)
http://arxiv.org/abs/0801.4572
Gas and Dust Emission at the Outer Edge of Protoplanetary Disks
Authors: A. M. Hughes, D. J. Wilner, C. Qi, M. R. Hogerheijde
(Submitted on 30 Jan 2008)
Abstract: We investigate the apparent discrepancy between gas and dust outer radii derived from millimeter observations of protoplanetary disks. Using 230 and 345 GHz continuum and CO J=3-2 data from the Submillimeter Array for four nearby disk systems (HD 163296, TW Hydrae, GM Aurigae, and MWC 480), we examine models of circumstellar disk structure and the effects of their treatment of the outer disk edge. We show that for these disks, models described by power laws in surface density and temperature that are truncated at an outer radius are incapable of reproducing both the gas and dust emission simultaneously: the outer radius derived from the dust continuum emission is always significantly smaller than the extent of the molecular gas disk traced by CO emission. However, a simple model motivated by similarity solutions of the time evolution of accretion disks that includes a tapered exponential edge in the surface density distribution (and the same number of free parameters) does much better at reproducing both the gas and dust emission. While this analysis does not rule out the disparate radii implied by the truncated power-law models, a realistic alternative disk model, grounded in the physics of accretion, provides a consistent picture for the extent of both the gas and dust.
Comments: 9 pages, 2 figures, accepted for publication in ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.4763v1 [astro-ph]
Submission history
From: Meredith Hughes [view email]
[v1] Wed, 30 Jan 2008 21:06:48 GMT (78kb)
http://arxiv.org/abs/0801.4763
Gas and Dust Associated with the Strange, Isolated, Star BP Piscium
Authors: B. Zuckerman, C. Melis, Inseok Song, David S. Meier, Marshall D. Perrin, Bruce Macintosh, Christian Marois, Alycia J. Weinberger, Joseph H. Rhee, James R. Graham, Joel H. Kastner, Patrick Palmer, T. Forveille, E.E. Becklin, D. J. Wilner, T. S. Barman, G. W. Marcy, M. S. Bessell
(Submitted on 2 Feb 2008)
Abstract: We have carried out a multiwavelength observational campaign demonstrating some of the remarkable properties of the infrared-bright variable star BP Psc. Surrounded by a compact dusty, gaseous disk, this little-studied late-G (or early-K) type star emits about 75% of its detected energy flux at infrared wavelengths. Evidence for accretion of gas in conjunction with narrow bi-polar jets and Herbig-Haro objects is apparently consistent with classification of BP Psc as a pre-main sequence star, as postulated in most previous studies.
If young, then BP Psc would be one of the nearest and oldest known classical T Tauri stars. However, such an evolutionary classification encounters various problems that are absent or much less severe if BP Psc is instead a luminosity class III post-main sequence star. In this case, it would be the first known example of a first ascent giant surrounded by a massive molecular disk with accompanying rapid gas accretion and prominent jets and HH objects.
In this model, the genesis of the massive dusty gaseous disk could be a consequence of the envelopment of a low mass companion star. Properties in the disk may be conducive to the current formation of planets, a gigayear or more after the formation of BP Psc itself.
Comments: Accepted for Astrophysical Journal
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0226v1 [astro-ph]
Submission history
From: Benjamin Zuckerman [view email]
[v1] Sat, 2 Feb 2008 00:14:39 GMT (969kb)
http://arxiv.org/abs/0802.0226
The Structure of Protoplanetary Disks Surrounding Three Young Intermediate Mass Stars. I. Resolving the disk rotation in the [OI] 6300 Å line
Authors: G. van der Plas (1,2), M. E. van den Ancker (1), D. Fedele (1,3,4), B. Acke (5), C. Dominik (2), L.B.F.M. Waters (2), J. Bouwman (4) ((1) European Southern Observatory, (2) University of Amsterdam, (3) Universita Degli Studi Di Padova, (4) Max-Planck-Institut fur Astronomie, Heidelberg, (5) Institute of Astronomy, KU Leuven)
(Submitted on 5 Feb 2008)
Abstract: We present high spectral resolution optical spectra of three young intermediate mass stars, in all of which we spectrally resolve the 6300 Angstrom [OI] emission line. Two of these have a double peaked line profile. We fit these data with a simple model of the [OI] emission caused by photo-dissociation of OH molecules in the upper layer of a circumstellar disk by stellar UV radiation and thus translate the Doppler broadened [OI] emission profile into an amount of emission as a function of distance from the central star.
The resulting spectra are in agreement with the expected disk shapes as derived from their spectral energy distribution. We find evidence for shadowing by an inner rim in the disk surrounding HD101412 and see a flaring disk structure in HD179218 while the [OI] spectrum of HD135344 is more complex. The [OI] emission starts for all three targets at velocities corresponding to their dust sublimation radius and extends up to radii of 10 — 90 AU. This shows that this method can be a valuable tool in the future investigation of circumstellar disks.
Comments: 11 pages, 19 figures, accepted in A&A on 04/02/2008; added references
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0707v1 [astro-ph]
Submission history
From: Gerrit van der Plas [view email]
[v1] Tue, 5 Feb 2008 21:19:12 GMT (735kb)
http://arxiv.org/abs/0802.0707
Observational Possibility of the “Snow Line” on the Surface of Circumstellar Disks with the Scattered Light
Authors: Akio K. Inoue (Osaka Sangyo Univ.), Mitsuhiko Honda (Kanagawa Univ.), Taishi Nakamoto (Tokyo Inst. Tech.), Akinori Oka (Tokyo Inst. Tech.)
(Submitted on 7 Feb 2008)
Abstract: We discuss how we obtain the spatial distribution of ice on the surface of the circumstellar disk around young stars. Ice in the disks plays a very important role in various issues, for instance, on the disk structure, on the planet formation, on the isotopic anomaly in meteorites, and on the origin of the sea on the Earth. Therefore, the spatially resolved observation of the condensation/sublimation front of ice, so-called “snow line” is strongly required. Here, we propose a new method for obtaining the spatially resolved “snow line” on the circumstellar disks by observing 3 \micron H$_2$O ice feature in the scattered light. Based on radiative transfer considerations, we show that the feature is clearly imprinted in the spectrum of the scattered light from both optically thick and thin circumstellar disks.
We also show that the scattered light and the H$_2$O ice feature from protoplanetary disks are detectable and spatially resolvable with the current instruments through a $H_2O$ narrowband filter around 3 \micron. Finally, we present a diagnostics of disk dust properties on the $K-H_2O$ and $K-L’$ two color diagram.
Comments: PASJ accepted, single column 16 pages, 5 figures
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0906v1 [astro-ph]
Submission history
From: Akio Inoue [view email]
[v1] Thu, 7 Feb 2008 03:20:26 GMT (63kb)
http://arxiv.org/abs/0802.0906
LkH$\alpha$ 330: Evidence for dust clearing through resolved submillimeter imaging
Authors: J.M Brown, G.A. Blake, C. Qi, C.P. Dullemond, D.J. Wilner
(Submitted on 7 Feb 2008)
Abstract: Mid-infrared spectrophotometric observations have revealed a small sub-class of circumstellar disks with spectral energy distributions (SEDs) suggestive of large inner gaps with low dust content. However, such data provide only an indirect and model dependent method of finding central holes. We present here the direct characterization of a 40 AU radius inner gap in the disk around LkHa 330 through 340 GHz (880 micron) dust continuum imaging with the Submillimeter Array (SMA). This large gap is fully resolved by the SMA observations and mostly empty of dust with less than 1.3 x 10^-6 M_solar of solid particles inside of 40 AU. Gas (as traced by accretion markers and CO M-band emission) is still present in the inner disk and the outer edge of the gap rises steeply — features in better agreement with the underlying cause being gravitational perturbation than a more gradual process such as grain growth. Importantly, the good agreement of the spatially resolved data and spectrophometry-based model lends confidence to current interpretations of SEDs with significant dust emission deficits as arising from disks with inner gaps or holes. Further SED-based searches can therefore be expected to yield numerous additional candidates that can be examined at high spatial resolution.
Comments: 11 pages, 3 figures, accepted to ApJL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.0998v1 [astro-ph]
Submission history
From: Joanna Brown [view email]
[v1] Thu, 7 Feb 2008 15:04:14 GMT (108kb)
http://arxiv.org/abs/0802.0998
Formation of simple organic molecules in inner T Tauri disks
Authors: Marcelino Agundez, Jose Cernicharo, Javier R. Goicoechea
(Submitted on 6 Mar 2008)
Abstract: We present time dependent chemical models for a dense and warm O-rich gas exposed to a strong far ultraviolet field aiming at exploring the formation of simple organic molecules in the inner regions of protoplanetary disks around T Tauri stars. An up-to-date chemical network is used to compute the evolution of molecular abundances. Reactions of H2 with small organic radicals such as C2 and C2H, which are not included in current astrochemical databases, overcome their moderate activation energies at warm temperatures and become very important for the gas phase synthesis of C-bearing molecules. The photodissociation of CO and release of C triggers the formation of simple organic species such as C2H2, HCN, and CH4. In timescales between 1 and 10,000 years, depending on the density and FUV field, a steady state is reached in the model in which molecules are continuously photodissociated but also formed, mainly through gas phase chemical reactions involving H2. The application of the model to the upper layers of inner protoplanetary disks predicts large gas phase abundances of C2H2 and HCN. The implied vertical column densities are as large as several 10^(16) cm^(-2) in the very inner disk (less than 1 AU), in good agreement with the recent infrared observations of warm C2H2 and HCN in the inner regions of IRS 46 and GV Tau disks. We also compare our results with previous chemical models studying the photoprocessing in the outer disk regions, and find that the gas phase chemical composition in the upper layers of the inner terrestrial zone (a few AU) is predicted to be substantially different from that in the upper layers of the outer disk (greater than 50 AU).
Comments: 7 pages; accepted for publication in A&A
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0803.0938v1 [astro-ph]
Submission history
From: Marcelino Agundez [view email]
[v1] Thu, 6 Mar 2008 18:38:49 GMT (212kb)
http://arxiv.org/abs/0803.0938
The nature of mid-infrared excesses from hot dust around Sun-like stars
Authors: R. Smith, M. C. Wyatt, W. R. F. Dent
(Submitted on 29 Apr 2008)
Abstract: (ABRIDGED) Studies of debris disks have shown that most systems are analogous to the Edgeworth-Kuiper Belt. However a rare subset of sun-like stars possess dust which lies in the terrestrial planet region.
In this study we aim to determine how many sources with apparent mid-IR excess are truly hosts of warm dust, and investigate where the dust must lie. We observed using mid-IR imaging with TIMMI2, VISIR and MICHELLE a sample of FGK main sequence stars reported to have hot dust. A new modelling approach was developed to determine the constraints that can be set on the radial extent of excess emission.
We confirm the presence of warm dust around 3 of the candidates (eta Corvi, HD145263 and HD202406), and present constraints on the emitting dust regions. Of 2 alternative models for the eta Corvi excess emission, we find that a model with 1 hot dust component at <3 AU (combined with the known submm dust population) fits the data better at the 2.6sigma level than an alternative model with 2 populations of dust in the mid-IR. We identify several systems which have a companion (HD65277 and HD79873) or background object (HD53246, HD123356 and HD128400) responsible for their mid-infrared excess, and for 3 other systems we were able to rule out a point-like source near the star at the level of excess observed in lower resolution observations (HD12039, HD69830 and HD191089).
Hot dust sources are either young and possibly primordial or transitional, or have relatively small radius steady-state planetesimal belts, or they are old and luminous with transient emission. High resolution imaging can be used to constrain the location of the disk and help to discriminate between different models of disk emission. For some small disks, interferometry is needed to resolve the disk location.
Comments: Accepted for publication in Astronomy & Astrophysics
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0804.4580v1 [astro-ph]
Submission history
From: Rachel Smith [view email]
[v1] Tue, 29 Apr 2008 11:23:50 GMT (408kb)
http://arxiv.org/abs/0804.4580
Molecules in the Circumstellar Disk Orbiting BP Piscium
Authors: Joel H. Kastner (Laboratoire d’Astrophysique de Grenoble and
Rochester Institute of Technology), B. Zuckerman (UCLA), Thierry Forveille (Laboratoire d’Astrophysique de Grenoble)
(Submitted on 15 May 2008)
Abstract: BP Psc is a puzzling late-type, emission-line field star with large infrared excess. The star is encircled and enshrouded by a nearly edge-on, dust circumstellar disk, and displays an extensive jet system similar to those associated with pre-main sequence (pre-MS) stars.
We conducted a mm-wave molecular line survey of BP Psc with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and, possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not detected. The CO line profiles of BP Psc are well fit by a model invoking a disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses.
The weakness of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+ and CN emission that characterizes most low-mass, pre-main sequence stars that have been the subjects of molecular emission-line surveys, and is suggestive of a very low level of X-ray-induced molecular ionization within the BP Psc disk. These results lend some support to the notion that BP Psc is an evolved star whose circumstellar disk has its origins in a catastrophic interaction with a close companion.
Comments: 6 pages, 4 figures; to appear in Astronomy & Astrophysics
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0805.2293v1 [astro-ph]
Submission history
From: Joel Kastner [view email]
[v1] Thu, 15 May 2008 13:05:17 GMT (36kb)
http://arxiv.org/abs/0805.2293
Migration of Protoplanets in Radiative Disks
Authors: Wilhelm Kley, Aurelien Crida (Universiy of Tuebingen)
(Submitted on 18 Jun 2008)
Abstract: In isothermal disks the migration of protoplanets is directed inward. For small planetary masses the standard type-I migration rates are so fast that this may result in an unrealistic loss of planets into the stars. We investigate the planet-disk interaction in non-isothermal disks and analyze the magnitude and direction of migration for an extended range of planet masses. We have performed detailed two-dimensional numerical simulations of embedded planets including heating/cooling effects as well as radiative diffusion for realistic opacities.
In radiative disks, small planets with M_planet < 50 M_Earth do migrate outward with a rate comparable to absolute magnitude of standard type-I migration. For larger masses the migration is inward and approaches the isothermal, type-II migration rate. Our findings are particularly important for the first growth phase of planets and ease the problem of too rapid inward type-I migration.
Comments: 4 Pages, accepted by AA Letters
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0806.2990v1 [astro-ph]
Submission history
From: Willy Kley [view email]
[v1] Wed, 18 Jun 2008 13:18:14 GMT (177kb)
http://arxiv.org/abs/0806.2990
STIS Imaging of the HR 4796A Circumstellar Debris Ring
Authors: G. Schneider, A. J. Weinberger, E. E. Becklin, J. H. Debes, B. A. Smith
(Submitted on 1 Oct 2008)
Abstract: We have obtained high spatial resolution imaging observations of the HR 4796A circumstellar debris dust ring using the broad optical response of the Hubble Space Telescope Imaging Spectrograph in coronagraphic mode.
We use our visual wavelength observations to improve upon the earlier measured geometrical parameters of the ring-like disk.
Two significant flux density asymmetries are noted: (1) preferential forward scattering by the disk grains and (2) an azimuthal surface brightness anisotropy about the morphological minor axis of the disk with corresponding differential ansal brightness.
We find the debris ring offset from the location of the star by ~1.4 AU, a shift insufficient to explain the differing brightnesses of the NE and SW ansae simply by the 1/$r^2$ dimmunition of starlight. The STIS data also better quantify the radial confinement of the starlight-scattering circumstellar debris, to a characteristic region <14 AU in photometric half-width, with a significantly steeper inner truncation than outward falloff in radial surface brightness.
The inferred spatial distribution of the disk grains is consistent with the possibility of one or more unseen co-orbital planetary-mass perturbers, and the colors of the disk grains are consistent with a collisionally evolved population of debris, possibly including ices reddened by radiation exposure to the central star.
Comments: Accepted to AJ; 3 of 8 figures in color
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0810.0286v1 [astro-ph]
Submission history
From: Alycia J. Weinberger [view email]
[v1] Wed, 1 Oct 2008 20:49:50 GMT (1011kb)
http://arxiv.org/abs/0810.0286