Peter R. Allen

THE DISK POPULATION OF THE TAURUS STAR-FORMING REGION*

We have analyzed nearly all images of the Taurus star-forming region at 3.6, 4.5, 5.8, 8.0, and 24??m that were obtained during the cryogenic mission of the Spitzer Space Telescope (46 deg2) and have measured photometry for all known members of the region that are within these data, corresponding to 348 sources, or 99% of the known stellar population. By combining these measurements with previous observations with the Spitzer Infrared Spectrograph and other facilities, we have classified the members of Taurus according to whether they show evidence of circumstellar disks and envelopes (classes I, II, and III). Through these classifications, we find that the disk fraction in Taurus, N(II)/N(II+III), is ~75% for solar-mass stars and declines to ~45% for low-mass stars and brown dwarfs (0.01-0.3 M ?). This dependence on stellar mass is similar to that measured for Chamaeleon I, although the disk fraction in Taurus is slightly higher overall, probably because of its younger age (1?Myr versus 2-3?Myr). In comparison, the disk fraction for solar-mass stars is much lower (~20%) in IC 348 and ? Ori, which are denser than Taurus and Chamaeleon I and are roughly coeval with the latter. These data indicate that disk lifetimes for solar-mass stars are longer in star-forming regions that have lower stellar densities. Through an analysis of multiple epochs of Spitzer photometry that are available for ~200 Taurus members, we find that stars with disks exhibit significantly greater mid-infrared (mid-IR) variability than diskless stars, which agrees with the results of similar variability measurements for a smaller sample of stars in Chamaeleon I. The variability fraction for stars with disks is higher in Taurus than in Chamaeleon I, indicating that the IR variability of disks decreases with age. Finally, we have used our data in Taurus to refine the observational criteria for primordial, evolved, and transitional disks. The ratio of the number of evolved and transitional disks to the number of primordial disks in Taurus is 15/98 for spectral types of K5-M5, indicating a timescale of 0.15 ? ?primordial ~ 0.45?Myr for the clearing of the inner regions of optically thick disks. After applying the same criteria to older clusters and associations (2-10?Myr) that have been observed with Spitzer, we find that the proportions of evolved and transitional disks in those populations are consistent with the measurements in Taurus when their star formation histories are properly taken into account.

Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field

We present a 20 pc, volume-limited sample of M7YL8 dwarfs created through spectroscopic follow-up of sources selected from the Two Micron All Sky Survey Second Incremental Release Point Source Catalog. In this paper we present optical spectroscopy of 198 candidate nearby ultracool dwarfs, including 12 late-M and L dwarfs likely to be within 20 pc of the Sun and 94 more distant late-type dwarfs. We have also identifiedfive ultracool dwarfs withspectral signatures of low gravity. Combining these data with previous results, we definea sample of 99ultracool dwarfs in 91 systems within 20 pc. These are used to estimate the J- and K-band luminosity functions for dwarfs with optical spectral types between M7 and L8 (10:5 < MJ < 15, 9:5 < MKS < 13). We find a space density of 4:9 ;10 � 3 pc � 3 for late-M dwarfs (M7YM9.5) and a lower limit of 3:8 ;10 � 3 pc � 3 for L dwarfs.

The Disk Population of the Chamaeleon I Star-forming Region*

We present a census of circumstellar disks in the Chamaeleon I star-forming region. Using the Infrared Array Camera and the Multiband Imaging Photometer on board the Spitzer Space Telescope, we have obtained images of Chamaeleon I at 3.6, 4.5, 5.8, 8.0, and 24 ?m. To search for new disk-bearing members of the cluster, we have performed spectroscopy on objects that have red colors in these data. Through this work, we have discovered four new members of Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three objects are highly embedded ( -->AJ ~ 5) and reside near known protostars, indicating that they may be among the youngest low-mass sources in the cluster ( -->? M 1 M? is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%), indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus, low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters.

Meeting the Cool Neighbors. X. Ultracool Dwarfs from the 2MASS All-Sky Data Release

Using data from the 2 Micron All Sky Survey All-Sky Point Source Catalogue, we have extended our census of nearby ultracool dwarfs to cover the full celestial sphere above Galactic latitude of 15°. Starting with an initial catalog of 2,139,484 sources, we have winnowed the sample to 467 candidate late-type M or L dwarfs within 20 pc of the Sun. Fifty-four of those sources already have spectroscopic observations confirming them as late-type dwarfs. We present optical spectroscopy of 376 of the remaining 413 sources, and identify 44 as ultracool dwarfs with spectroscopic distances less than 20 pc. Twenty-five of the 37 sources that lack optical data have near-infrared spectroscopy. Combining the present sample with our previous results and data from the literature, we catalog 94 L dwarf systems within 20 pc. We discuss the distribution of activity, as measured by Hα emission, in this volume-limited sample. We have coupled the present ultracool catalog with data for stars in the northern 8 pc sample and recent (incomplete) statistics for T dwarfs to provide a snapshot of the current 20 pc census as a function of spectral type.

AN INFRARED/X-RAY SURVEY FOR NEW MEMBERS OF THE TAURUS STAR-FORMING REGION ∗

We present the results of a search for new members of the Taurus star-forming region using data from the Spitzer Space Telescope and the XMM-Newton Observatory. We have obtained optical and near-infrared spectra of 44 sources that exhibit red Spitzer colors that are indicative of stars with circumstellar disks and 51 candidate young stars that were identified by Scelsi and coworkers using XMM-Newton. We also performed spectroscopy on four possible companions to members of Taurus that were reported by Kraus and Hillenbrand. Through these spectra, we have demonstrated the youth and membership of 41 sources, 10 of which were independently confirmed as young stars by Scelsi and coworkers. Five of the new Taurus members are likely to be brown dwarfs based on their late spectral types (>M6). One of the brown dwarfs has a spectral type of L0, making it the first known L-type member of Taurus and the least massive known member of the region (M ~ 4-7 M_(Jup)). Another brown dwarf exhibits a flat infrared spectral energy distribution, which indicates that it could be in the protostellar class I stage (star+disk+envelope). Upon inspection of archival images from various observatories, we find that one of the new young stars has a large edge-on disk (r = 25 = 350 AU). The scattered light from this disk has undergone significant variability on a timescale of days in optical images from the Canada-France-Hawaii Telescope. Using the updated census of Taurus, we have measured the initial mass function for the fields observed by XMM-Newton. The resulting mass function is similar to previous ones that we have reported for Taurus, showing a surplus of stars at spectral types of K7-M1 (0.6-0.8 M ) relative to other nearby star-forming regions, such as IC 348, Chamaeleon I, and the Orion Nebula Cluster.

Star Formation via the Little Guy: A Bayesian Study of Ultracool Dwarf Imaging Surveys for Companions

I have undertaken a comprehensive statistical investigation of the ultracool dwarf companion distribution (spectral type M6 and later). Utilizing a Bayesian algorithm, I tested models of the companion distribution against data from an extensive set of space and ground-based imaging observations of nearby ultracool dwarfs. My main conclusions are fivefold: (1) confirm that the concentration of high mass ratio ultracool binary systems is a fundamental feature of the companion distribution, not an observational or selection bias; (2) determine that the wide (>~20 AU) binary frequency can be no more than 1%-2%; (3) show that the decreasing binary frequency with later spectral types is a real trend; (4) demonstrate that a large population of currently undetected low mass ratio systems are not consistent with the current data; and (5) find that the population of spectroscopic binaries must be at least 30% that of currently known ultracool binaries. The best-fit value for the overall M6 and later binary frequency is ~20%-22%, of which only ~6% consists of currently undetected companions with separations less than 1 AU. If this is correct, then the upper limit of the ultracool binary population discovered to date is ~75%. I find that the numerical simulation results of the ejection formation method are inconsistent with the outcome of this analysis. However, dynamics do seem to play an important role, as simulations of small-N clusters and triple system decays produce results similar to those of this work. The observational efforts required to improve these constraints are shown to be primarily large spectroscopic binary surveys and improved high-resolution imaging techniques.

A Search for Binary Systems among the Nearest L Dwarfs

We have used the Near-Infrared Camera and Multi-Object Spectrometer NIC1 camera on the Hubble Space Telescopeto obtain high angular resolution images of 52 ultracool dwarfs in the immediate solar neighborhood. Nine systems are resolved as binary, with component separations from 1.5 to 15 AU. Based on current theoretical models and empirical bolometric corrections, allsystemshave components with similar luminosities and, consequently,high mass ratios, q > 0:8. Limiting analysis to L dwarfs within 20 pc, the observed binary fraction is 12% þ7 � 3 . Applying Bayesian analysis to our data set, we derive a mass-ratio distribution that peaks strongly at unity. Modeling the semimajor axis distribution as a logarithmic Gaussian, the best fit is centered at loga0 ¼ 0: 8A U (� 6.3 AU), with a (logarithmic) width of � 0.3. The current data are consistent with an overall binary frequency of � 24%.

MEETING THE COOL NEIGHBORS. XI. BEYOND THE NLTT CATALOG

Visiting Astronomer, Kitt Peak National Observatory, NOAO, which is operated by AURA under cooperative agreement with the NSF. Spitzer FellowWe have combined near-infrared data from the Two Micron All Sky Survey with ancillary optical data to identify previously unrecognized proper-motion stars that have colors and magnitudes consistent with nearby M dwarfs. We present follow-up observations of 392 stars from that sample, including ~200 stars discussed here for the first time. Our distance estimates, based primarily on spectroscopic parallaxes, place 123 stars within 20 pc of the Sun. One hundred and seventy-six stars exhibit Hα emission, and 82 stars have plausible X-ray counterparts from ROSAT observations. The distribution of chromospheric and coronal activity is broadly consistent with data for other nearby late-type stars, with ~30 stars having activity levels comparable to Pleiades M dwarfs. We have identified several M dwarfs that are probably wide ( >1000 AU) companions of F and G stars. We have recomputed the 20 pc luminosity function, adding the 123 new candidates found in the present survey. The revised luminosity function is in closer agreement with the 8 pc data set for stars with MJ > 8 but probably remains incomplete at the ~20% level.

New Low-Mass Stars and Brown Dwarfs with Disks in Lupus

Using the Infrared Array Camera and the Multiband Imaging Photometer aboard the Spitzer Space Telescope, we have obtained images of the Lupus 3 star-forming cloud at 3.6, 4.5, 5.8, 8.0, and 24 μm. We present photometry in these bands for the 41 previously known members that are in our images. In addition, we have identified 19 possible new members of the cloud based on red 3.6-8.0 μm colors that are indicative of circumstellar disks. We have performed optical spectroscopy on six of these candidates, all of which are confirmed as young low-mass members of Lupus 3. The spectral types of these new members range from M4.75 to M8, corresponding to masses of 0.2-0.03 M☉ for ages of ~1 Myr according to theoretical evolutionary models. We also present optical spectroscopy of a candidate disk-bearing object in the vicinity of the Lupus 1 cloud, 2M 1541-3345, which Jayawardhana & Ivanov recently classified as a young brown dwarf (M ~ 0.03 M☉) with a spectral type of M8. In contrast to their results, we measure an earlier spectral type of M5.75 ± 0.25 for this object, indicating that it is probably a low-mass star (M ~ 0.1 M☉). In fact, according to its gravity-sensitive absorption lines and its luminosity, 2M 1541-3345 is older than members of the Lupus clouds (τ ~ 1 Myr) and instead is probably a more evolved pre-main-sequence star that is not directly related to the current generation of star formation in Lupus.

SPITZER/IRAC SEARCH FOR COMPANIONS TO NEARBY, YOUNG M DWARFS

We present the results of a survey of nearby, young M stars for wide low-mass companions with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observed 40 young M dwarfs within 20 pc of the Sun, selected through X-ray emission criteria. A total of ten candidate companions were found with IRAC colors consistent with T dwarfs. Extensive ground-based NIR follow-up observations rejected all these candidates. Two additional candidates were discovered via common proper motion measurements, one of which was rejected as a background object and the other is a bona fide companion to GJ 2060, a member of the AB Doradus moving group.