Neon isn’t an unusual find in spectroscopic studies of massive stars or, for that matter, in observations of novae or the galactic core. Energetic X-ray or ultraviolet emissions can ionize the gas, at which point it produces infrared light at characteristic wavelengths. Not expecting to find it around low-mass stars like our Sun, researchers have been surprised to find four Sun-like stars showing neon in their disks as measured by a Spitzer Space Telescope project run by the University of Arizona.
That project, called Formation and Evolution of Planetary Systems (FEPS) is run out of Steward Observatory. The idea is to study planet-forming gas around 35 young, solar-type stars. Before this work, none of these stars would have been thought energetic enough to radiate the amount of X-ray and ultraviolet light needed to ionize neon.
Unexpected though they might be, the observations are useful because neon, while hardly abundant, offers a precise spectral signature that makes it easier for astronomers to track the gas content in circumstellar disks. The neon signature, says Michael R. Meyer (Steward Observatory), reminds us that the planet formation period can be a violent one. And future detections of the gas may help to differentiate between stars retaining their dust and gas disks and those that have already begun to form planets.
“It serves as a valuable reminder that the environment in which the planets formed was harsh, too, in a way. And it could turn out to be an important tracer of remnant gas in circumstellar disks — a kind of ‘vacancy/no vacancy’ sign for planets.”
Image: In this artist’s conception, a possible newfound planet spins through a clearing in a nearby star’s dusty, planet-forming disk. (Image credit: NASA/JPL/R. Hurt, SSC).
If so, neon makes a nice fixture in the toolkit of researchers anticipating the launch of future planet-hunter missions like Kepler and ESA’s Darwin. The more methods we can bring to bear in choosing targets for precious telescope time, the better our chances of success. Tracking stars whose inner disks show gases that can circularize the orbits of Earth-like planets will speed the process. Future studies of stellar systems of different ages are being developed to show how the gas dissipates with time.
The paper is Pascucci, “Detection of [Ne II] Emission from Young Circumstellar Disks,” The Astrophysical Journal 663 (July 1, 2007), pp. 383-393 (abstract).
I’m not so sure targetting systems that look likely to have circular planets is a good idea. Firstly. eccentric terrestrial planet systems would be fairly challenging to current theories (as far as I can tell). Secondly, an eccentric Earth near the habitable zone could have some fairly interesting climates… the habitability of such worlds is a far more interesting question (IMHO) than the quest for Earth-analogues. Besides, we already know Earth-analogues can support life: not so for eccentric ones.
Me, I think we don’t know squat. Target anything we can observe with reasonable resource and effort.
Current # of the 10exp 12 or so stars and planet in this galaxy we know have life = 0.
Not 0. ONE! :)
One? This is too low and allow approximation errors. This give birth to, for example Rare Earth claims.
0, 1, an infinite number of orders of magnitude…
The Complete Census of 70-um-Bright Debris Disks within the FEPS (Formation and Evolution of Planetary Systems) Spitzer Legacy Survey of Sun-like Stars
Authors: Lynne A. Hillenbrand, John M. Carpenter, Jinyoung Serena Kim, Michael R. Meyer, Dana E. Backman, Amaya Moro-Martin, David J. Hollenbach, Dean C. Hines, Ilaria Pascucci, Jeroen Bouwman
(Submitted on 30 Dec 2007)
Abstract: (abbreviated) We report detection with the Spitzer Space Telescope of cool dust surrounding solar type stars. The observations were performed as part of the Legacy Science Program, “Formation and Evolution of Planetary Systems” (FEPS). From the overall FEPS sample (Meyer et al. 2006) of 328 stars having ages ~0.003-3 Gyr we have selected sources with 70 um flux densities indicating excess in their spectral energy distributions above expected photospheric emission…….. …..The rising spectral energy distributions towards – and perhaps beyond – 70 um imply dust temperatures T_dust less than 45-85 K for debris in equilibrium with the stellar radiation field. We infer bulk properties such as characteristic temperature, location, fractional luminosity, and mass of the dust from fitted single temperature blackbody models. For greater than 1/3 of the debris sources we find that multiple temperature components are suggested, implying a spatial distribution of dust extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well.
Preliminary assessment of the statistics of cold debris around sun-like stars shows that ~10% of FEPS targets with masses between 0.6 and 1.8 Msun and ages between 30 Myr and 3 Gyr exhibit 70 um emission in excess of the expected photospheric flux density. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 um excess frequency with stellar mass.
Comments: article accepted to ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.0163v1 [astro-ph]
From: Lynne Hillenbrand [view email]
[v1] Sun, 30 Dec 2007 20:05:57 GMT (276kb)
The formation and evolution of planetary systems: Grain growth and chemical processing of dust in T Tauri systems
Authors: J. Bouwman, Th. Henning, L.A. Hillenbrand, M.R. Meyer, I. Pascucci, J. Carpenter, D. Hines, J.S. Kim, M.D. Silverstone, D. Hollenbach, S. Wolf
(Submitted on 21 Feb 2008)
Abstract: This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium, and show evidence for a non-negligible (~5 % in mass fractions) contribution from crystalline species.
These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains towards the disk mid-plane. Further, we find a change in the relative abundance of the different crystalline species: more enstatite relative to forsterite is observed in the inner warm dust population at ~1 AU, while forsterite dominates in the colder outer regions at ~5 to 15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario where crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk.
Last, we report the detection of emission from polycyclic aromatic hydrocarbon molecules in five out of seven sources. We find a tentative PAH band at 8.2 micron, previously undetected in the spectra of disks around low-mass pre-main-sequence stars.
Comments: 53 pages, 14 figures, 3 tables, ApJ accepted
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.3033v1 [astro-ph]
From: Jeroen Bouwman [view email]
[v1] Thu, 21 Feb 2008 12:51:57 GMT (271kb)