Authors: K. L. Luhman, A. A. Muench

(Submitted on 23 May 2008)

Abstract: We have used images obtained with the Infrared Array Camera and the Multiband Imaging Photometer onboard the Spitzer Space Telescope to search for low-mass stars and brown dwarfs with circumstellar disks in the Chamaeleon I star-forming region. Through optical spectroscopy of sources with red colors in these data, we have identified seven new disk-bearing members of the cluster.

Three of these objects are probably brown dwarfs according to their spectral types (M8, M8.5, M8-L0). Three of the other new members may have edge-on disks based on the shapes of their infrared spectral energy distributions. One of the possible edge-on systems has a steeply rising slope from 4.5 to 24um, indicating that it could be a class I source (star+disk+envelope) rather than a class II source (star+disk). If so, then it would be one of the least massive known class I protostars (M5.75, M~0.1 Msun).

Comments: Astrophysical Journal, in press

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.3722v1 [astro-ph]

Submission history

From: Kevin Luhman [view email]

[v1] Fri, 23 May 2008 21:29:38 GMT (947kb)

http://arxiv.org/abs/0805.3722

Authors: Ch. Helling, P. Woitke, W.-F. Thi

(Submitted on 30 Mar 2008)

Abstract: We aim at understanding the formation of cloud layers in quasi-static substellar atmospheres. The time-dependent description presented in (Helling & Woitke 2006) is a kinetic model describing nucleation, growth and evaporation. It is extended to treat gravitational settling and is applied to the static-stationary case of substellar model atmospheres. From the solution for the dust moments, we determine the grain size distribution function which, together with the calculated material volume fractions, provides the basis to calculate the opacities of the composite dust grains. The cloud particles in brown dwarfs and hot giant-gas planets are found to be small in the high atmospheric layers (0.01mum), and composed of a rich mixture of all considered condensates, in particular the abundant MgSiO3[s], Mg2SiO4[s] and SiO2[s]. As the particles settle downward, they increase in size and reach several 100mum in the deepest layers. The more volatile parts of the grains evaporate and the particles stepwise purify to form composite particles of high-temperature condensates in the deeper layers, mainly Fe[s] and Al2O3[s]. The gas phase abundances of the elements involved in the dust formation process vary by orders of magnitudes throughout the atmosphere. The grain size distribution is found to be relatively broad in the upper atmospheric layers but often strongly peaked in the deeper layers. The spectral appearance of the cloud layers in the mid IR (7-20mum) is close to a grey body with only weak broad features on a few percent level, mainly caused by MgSiO3[s], and Mg2SiO4[s]. Our models predict that the gas phase depletion is much weaker as compared to phase-equilibrium calculations in the high atmospheric layers. [abridged]

Comments: 16 pages, 9 Figures. A&A accepted

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.4315v1 [astro-ph]

Submission history

From: Christiane Helling [view email]

[v1] Sun, 30 Mar 2008 12:05:48 GMT (242kb)

http://arxiv.org/abs/0803.4315

Authors: H. Bouy, N. Huelamo, C. Pinte, J. Olofsson, D. Barrado y Navascues, E. L. Martin, E. Pantin, J.-L. Monin, G. Basri, J.-C. Augereau, F. Menard, G. Duvert, G. Duchene, F. Marchis, A. Bayo, S. Bottinelli, B. Lefort, S. Guieu

(Submitted on 13 Mar 2008)

Abstract: In order to improve our understanding of substellar formation, we have performed a compositional and structural study of a brown dwarf disk.
We present the result of photometric, spectroscopic and imaging observations of 2MASS J04442713+2512164, a young brown dwarf (M7.25) member of the Taurus association. Our dataset, combined with results from the literature, provides a complete coverage of the spectral energy distribution from the optical to the millimeter including the first photometric measurement of a brown dwarf disk at 3.7mm, and allows us to perform a detailed analysis of the disk properties.

The target was known to have a disk. High resolution optical spectroscopy shows that it is intensely accreting, and powers a jet and an outflow. The disk structure is similar to that observed for more massive TTauri stars. Spectral decomposition models of Spitzer/IRS spectra suggest that the mid-infrared emission from the optically thin disk layers is dominated by grains with intermediate sizes (1.5micron). Crystalline silicates are significantly more abondant in the outer part and/or deeper layers of the disk, implying very efficient mixing and/or additional annealing processes. Sub-millimeter and millimeter data indicate that most of the disk mass is in large grains (>1mm)

Comments: 17 pages, 10 figures, 7 tables, accepted for A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.2051v1 [astro-ph]

Submission history

From: Herve Bouy [view email]

[v1] Thu, 13 Mar 2008 21:00:38 GMT (313kb)

http://arxiv.org/abs/0803.2051

Authors: Mark S. Marley (NASA ARC), S.K. Leggett (Gemini Observatory)

(Submitted on 10 Mar 2008)

Abstract: Ultracool dwarfs exhibit a remarkably varied set of characteristics which hint at the complex physical processes acting in their atmospheres and interiors. Spectra of these objects not only depend upon their mass and effective temperature, but also their atmospheric chemistry, weather, and dynamics. As a consequence divining their mass, metallicity and age solely from their spectra has been a challenge. JWST, by illuminating spectral blind spots and observing objects with constrained masses and ages should finally unearth a sufficient number of ultracool dwarf Rosetta Stones to allow us to decipher the processes underlying the complex brown dwarf cooling sequence. In addition the spectra of objects invisible from the ground, including very low mass objects in clusters and nearby cold dwarfs from the disk population, will be seen for the first time. In combination with other ground- and space-based assets and programs, JWST will usher in a new golden era of brown dwarf science and discovery.

Comments: 23 pages, 9 figures, for publication in proceedings of “Astrophysics in the Next Decade: JWST and Concurrent Facilities”, a meeting held Sept. 24-27 in Tucson, Arizona

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.1476v1 [astro-ph]

Submission history

From: Mark S. Marley [view email]

[v1] Mon, 10 Mar 2008 19:23:50 GMT (2157kb,D)

http://arxiv.org/abs/0803.1476

Authors: Philippe Delorme (LAOG), Xavier Delfosse (LAOG, OSUG), Loic Albert (CFHT), Etienne. Artigau, Thierry Forveille (LAOG, OSUG, CFHT), Céline Reylé (LAOB), France Allard (CRAL), Derek Homeier, Annie Robin (LAOB), Chris J. Willott, Michael Liu (IfA), Trent Dupuy (IfA)
(Submitted on 29 Feb 2008 (v1), last revised 3 Mar 2008 (this version, v2))

Abstract: We report the discovery of CFBDS J005910.90-011401.3 (hereafter CFBDS0059), the coolest brown dwarf identified to date. We found CFBDS0059 using i’ and z’ images from the Canada-France-Hawaii Telescope (CFHT), and present optical and near-infrared photometry, Keck laser guide star adaptive optics imaging, and a complete near-infrared spectrum, from 1.0 to 2.2 $\mu$m. A side to side comparison of the near-infrared spectra of CFBDS0059 and ULAS~J003402.77-005206.7 (hereafter ULAS0034), previously the coolest known brown dwarf, indicates that CFBDS0059 is ~50+/-15K cooler. We estimate a temperature of Teff~620K and gravity of log g ~ 4.75. Evolutionary models translate these parameters into an age of 1-5 Gyr and a mass of 15-30 M_Jup. We estimate a photometric distance of ~13pc, which puts CFBDS0059 within easy reach of accurate parallax measurements. Its large proper motion suggests membership in the older population of the thin disk. The spectra of both CFBDS0059 and ULAS~J0034 shows probable absorption by a wide ammonia band on the blue side of the $H$-band flux peak. If, as we expect, that feature deepens further for still lower effective temperatures, its appearance will become a natural breakpoint for the transition between the T spectral class and the new Y spectral type. CFBDS0059 and ULAS~J0034 would then be the first Y0 dwarfs.

Comments: 12 pages, Submitted to A\&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.4387v2 [astro-ph]

Submission history

From: Philippe Delorme [view email] [via CCSD proxy]

[v1] Fri, 29 Feb 2008 13:20:57 GMT (881kb)

[v2] Mon, 3 Mar 2008 10:34:11 GMT (737kb)

http://arxiv.org/abs/0802.4387

You make a good point. If we could utilize brown dwarfs as realestate, there is no reason why ETI with more advanced technology might not have already laid claims to and have utilized them. For some dumb reason, I overlooked this fact.

Thanks;

Jim

It occurred to me that brown dwarfs upon full formation might produce enough black body thermal emmissions to provide natural warmth for very large space habitats that might be constructed in near orbit around them. Perhaps in order to maintain the warmth generated by these bodies, a gradual supply of very long lived radio nucleids could be dropped into the brown dwarf gradually and continuosly or approximately so inorder to provide warmth.

Uranium 238 is one isotope that comes to mind and it is U-238 which is believed to be largely responsible for the maintainence of the hot interior of the Earth which would have cooled billions of years ago were in not for the presence or radioactive isotopes within the Earth’s core and perhaps lower mantel.

Even longer lived isotopes might extend the length time that such brown dwarfs could provide heat. Brown dwarfs either by themselves or modified with radioactive material might be ideal stellar like space colony heat providers for future humanity because these dwarfs might not be under the political juristdictions of any ETI races. Any ETI civilizations might see such brown dwarfs as unwanted or un-needed real estate.

Thanks;

Jim

Authors: B. Riaz, J. E. Gizis

(Submitted on 14 Feb 2008)

Abstract: In our analysis of {\it Spitzer}/IRS archival data on the stellar and sub-stellar members of the TW Hydrae Association (TWA), we have discovered two new brown dwarf disks: a flat optically thick disk around SSSPM J1102-3431 (SSSPM 1102), and a transition disk around 2MASS J1139511-315921 (2M1139). The disk structure for SSSPM 1102 is found to be very similar to the known brown dwarf disk 2MASSW J1207334-393254 (2M1207), with excess emission observed at wavelengths as short as 5 $\micron$. 2M1139 shows no excess emission shortward of $\sim$20 $\micron$, but flares up at longer wavelengths, and is the first transition disk detected among the sub-stellar members of TWA. We also report on the {\it Spitzer}/70 $\micron$ observations, and the presence of a weak {\it absorption} 10 $\micron$ silicate feature for 2M1207. The absorption can be attributed to a close to edge-on disk at a 75$\degr$ inclination. The 10 $\micron$ spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 $\micron$. No silicate absorption/emission is observed towards SSSPM 1102. While only 6 out of 25 stellar members show excess emission at these mid-infrared wavelengths, {\it all} of the TWA brown dwarfs that have been observed so far with {\it Spitzer} show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at a relatively older age of $\sim$10 Myr. A comparison with younger clusters indicates that by the age of the TWA ($\sim$10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of $\sim$2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars.

Comments: 23 pages, 6 figures. Accepted in ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.2048v1 [astro-ph]

Submission history

From: Basmah Riaz [view email]

[v1] Thu, 14 Feb 2008 16:31:02 GMT (172kb)

http://arxiv.org/abs/0802.2048