Basmah Riaz

New Brown Dwarf Disks in the TW Hydrae Association

In our analysis of Spitzer IRS archival data on the stellar and substellar 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 μm. No excess emission shortward of ~20 μm is seen from 2M1139, but it flares up at longer wavelengths and is the first transition disk detected among the substellar members of the TWA. We also report on Spitzer 70 μm observations and the presence of a 10 μm silicate absorption feature for 2M1207. The absorption can be attributed to a nearly edge-on disk, at 75° inclination. The 10 μm spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 μm. No silicate absorption or emission is observed toward SSSPM 1102. While only six of 25 stellar members show excess emission at these mid-infrared wavelengths, all of the TWA brown dwarfs that have been observed so far with Spitzer show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at the relatively old age of ~10 Myr. A comparison with younger clusters indicates that by the age of the TWA (~10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of ~2 for the higher mass stars. This suggests longer disk decay timescales for brown dwarfs as compared with higher mass stars.

2006 WHOLE EARTH TELESCOPE OBSERVATIONS OF GD358: A NEW LOOK AT THE PROTOTYPE DBV

We report on the analysis of 436.1 hr of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 and 4000 μHz, with the dominant peak at 1234 μHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l = 1 modes in the limit of slow rotation. The high-k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high-k multiplets is highly suspect. The k = 9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high-k modes. Analysis of the k = 9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

Spitzer Observations of Nearby M Dwarfs

We present Spitzer IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have SEDs that are fitted within the error bars by a purely photospheric spectrum out to 24 μm. We find no evidence for IR excess. None of our targets are detected in the MIPS 70 and 160 μm bands. The estimated ages for all are >10 Myr, suggesting that enough disk dissipation has occurred within the inner several AU of the star. For four of these, Mullan et al. had reported IRAS detections at 12 μm, although the reported fluxes were below the 5 σ IRAS detection limit (~0.2 Jy). Mullan et al. also pointed out that V-K colors in dMe stars are larger than those in dM stars, possibly because of the presence of a chromosphere. Here we suggest that metallicity effects provide a better explanation of the V-K data. For reasons of observational selection, our targets are not the most active flare stars known, but being dMe stars indicates the presence of a chromosphere. Scaling from Houdebines model of the AU Mic chromosphere, we have computed the free-free IR excesses for a range of densities. Our Spitzer 24 μm data show that the chromospheres in two of our targets are less dense than in AU Mic by a factor of 10 or more. This is consistent with the fact that our sample includes the less active flare stars. Our models also indicate that the chromospheric contribution to the observed AU Mic emission at submillimeter wavelengths is only about 2%.