B- and A-Type Stars in the Taurus-Auriga Star Forming Region

We describe the results of a search for early-type stars associated with the Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region noted as lacking young stars of intermediate and high mass. We investigate several sets of possible O, B and early A spectral class members. The first is a group of stars for which mid-infrared images show bright nebulae, all of which can be associated with stars of spectral type B. The second group consists of early-type stars compiled from (i) literature listings in SIMBAD; (ii) B stars with infrared excesses selected from the Spitzer Space Telescope survey of the Taurus cloud; (iii) magnitude- and color-selected point sources from the 2MASS; and (iv) spectroscopically identified early-type stars from the SDSS coverage of the Taurus region. We evaluated stars for membership in the Taurus-Auriga star formation region based on criteria involving: spectroscopic and parallactic distances, proper motions and radial velocities, and infrared excesses or line emission indicative of stellar youth. For selected objects, we also model the scattered and emitted radiation from reflection nebulosity and compare the results with the observed spectral energy distributions to further test the plausibility of physical association of the B stars with the Taurus cloud. This investigation newly identifies as probable Taurus members three B-type stars: HR 1445 (HD 28929), tau Tau (HD 29763), 72 Tau (HD 28149), and two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5 or earlier associated with the Taurus clouds. Several additional early-type sources including HD 29659 and HD 283815 meet some, but not all, of the membership criteria and therefore are plausible, though not secure, members.Comment: 31 pages, 18 figures, 6 tables. Accepted for publication in The Astrophysical Journa

Ysovar: The First Sensitive, Wide-area, Mid-infrared Photometric Monitoring of the Orion Nebula Cluster

We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs

Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster

We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron) and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo) occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our photometry is complete for source detection at the 7 sigma level to K~17.5 mag and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual extinction values as high as 10 magnitudes. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/sub-stellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal. Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc

The photometric evolution of dissolving star clusters I: First predictions

We calculated the broad-band photometric evolution of unresolved star clusters, including the preferential loss of low-mass stars due to mass segregation. The stellar mass function of a cluster evolves due to three effects: (a) the evolution of massive stars; (b) early tidal effects reduce the mass function independently of the stellar mass; (c) after mass segregation has completed, tidal effects preferentially remove the lowest-mass stars from the cluster. Results: (1) During the first ~40% of the lifetime of a cluster the cluster simply gets fainter due to the loss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of low-mass stars. This will result in an underestimate of the age of clusters if standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of the total lifetime of a cluster it will rapidly get redder. This is because stars at the low-mass end of the main sequence, which are preferentially lost, are bluer than the AGB stars that dominate the light at long wavelengths, resulting in an age overestimate. (4) Clusters with mass segregation and the preferential loss of low-mass stars evolve along almost the same tracks in colour-colour diagrams as clusters without mass segregation. Therefore it will be difficult to distinguish this effect from that due to the cluster age for unresolved clusters, unless the total lifetime of the clusters can be estimated. (5) The changes in the colour evolution of unresolved clusters due to the preferential loss of low-mass stars will affect the determination of the SFHs. (6) The preferential loss of low-mass stars might explain the presence of old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as intermediate-age clusters (2 - 5 Gyr). [Abridged]Comment: accepted for publication in A&

UV excess measures of accretion onto young very low-mass stars and brown dwarfs

Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A leads to a higher contrast between the accretion and photospheric emission relative to higher-mass counterparts. The mass accretion rates measured here are systematically 4-7 times larger than those from H-alpha emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with H-alpha 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for accretors in the Taurus Molecular Cloud.Comment: 13 pages text, 15 tables, 14 figures. Accepted by Ap

Multiplicity and Optical Excess Across the Substellar Boundary in Taurus

We present the results of a high-resolution imaging survey of 22 brown dwarfs and very low mass stars in the nearby (~145 pc) young (~1-2 Myr) low-density star-forming region Taurus-Auriga. We obtained images with the Advanced Camera for Surveys/High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey confirmed the binarity of MHO-Tau-8 and discovered a new candidate binary system, V410-Xray3, resulting in a binary fraction of 9+/-5% at separations >4 AU. Both binary systems are tight (<10 AU) and they possess mass ratios of 0.75 and 0.46, respectively. The binary frequency and separations are consistent with low-mass binary properties in the field, but the mass ratio of V410-Xray3 is among the lowest known. We find that the binary frequency is higher for very low mass stars and high-mass brown dwarfs than for lower-mass brown dwarfs, implying either a decline in frequency or a shift to smaller separations for the lowest mass binaries. Combining these results with multiplicity statistics for higher-mass Taurus members suggests a gradual decline in binary frequency and separation toward low masses. The implication is that the distinct binary properties of very low-mass systems are set during formation and that the formation process is similar to the process which creates higher-mass stellar binaries, but occurs on a smaller scale. We show that there are no planets or very low-mass brown dwarfs with mass >3 M_J at projected separation >40 AU orbiting any of the Taurus members in our sample. We identify several BDs with significant (>1 mag) V-band excesses. The excesses appear to be correlated with signatures of accretion, and if attributed to accretion luminosity, may imply mass accretion rates several orders of magnitude above those inferred from line-profile analyses. (abridged)Comment: Accepted for publication in ApJ; 15 pages, 8 figures in emulateapj forma