Spitzer observations of the Orion OB1 association: disk census in the low mass stars

We present new Spitzer Space Telescope observations of two fields in the Orion OB1 association. We report here IRAC/MIPS observations for 115 confirmed members and 41 photometric candidates of the ~10 Myr 25 Orionis aggregate in the OB1a subassociation, and 106 confirmed members and 65 photometric candidates of the 5 Myr region located in the OB1b subassociation. The 25 Orionis aggregate shows a disk frequency of 6% while the field in the OB1b subassociation shows a disk frequency of 13%. Combining IRAC, MIPS and 2MASS photometry we place stars bearing disks in several classes: stars with optically thick disks (class II systems), stars with an inner transitional disks (transitional disk candidates) and stars with "evolved disks"; the last exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify 1 transitional disk candidate in the 25 Orionis aggregate and 3 in the OB1b field; this represents ~10% of the disk bearing stars, indicating that the transitional disk phase can be relatively fast. We find that the frequency of disks is a function of the stellar mass, suggesting a maximum around stars with spectral type M0. Comparing the infrared excess in the IRAC bands among several stellar groups we find that inner disk emission decays with stellar age, showing a correlation with the respective disk frequencies. The disk emission at the IRAC and MIPS bands in several stellar groups indicates that disk dissipation takes place faster in the inner region of the disks. Comparison with models of irradiated accretion disks, computed with several degrees of settling, suggests that the decrease in the overall accretion rate observed in young stellar groups is not sufficient to explain the weak disk emission observed in the IRAC bands for disk bearing stars with ages 5 Myr or older.Comment: Accepted in the Astrophysical Journa

Accretion properties of T Tauri stars in sigma Ori

Accretion disks around young stars evolve in time with time scales of few million years. We present here a study of the accretion properties of a sample of 35 stars in the ~3 million year old star-forming region sigma Ori. Of these, 31 are objects with evidence of disks, based on their IR excess emission. We use near-IR hydrogen recombination lines (Pa_gamma) to measure their mass accretion rate. We find that the accretion rates are significantly lower in sigma Ori than in younger regions, such as rho-Oph, consistently with viscous disk evolution. The He I 1.083 micron line is detected (either in absorption or in emission) in 72% of the stars with disks, providing evidence of accretion-powered activity also in very low accretors, where other accretion indicators dissapear.Comment: Astronomy and Astrophysics, accepte

The Formation and Early Evolution of Low-mass Stars and Brown Dwarfs

The discovery of large numbers of young low-mass stars and brown dwarfs over the last decade has made it possible to investigate star formation and early evolution in a previously unexplored mass regime. In this review, we begin by describing surveys for low-mass members of nearby associations, open clusters, star-forming regions and the methods used to characterize their stellar properties. We then use observations of these populations to test theories of star formation and evolution at low masses. For comparison to the formation models, we consider the initial mass function, stellar multiplicity, circumstellar disks, protostellar characteristics, and kinematic and spatial distributions at birth for low-mass stars and brown dwarfs. To test the evolutionary models, we focus on measurements of dynamical masses and empirical Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.Comment: Final published version at http://www.annualreviews.org/journal/astr

17 new very low-mass members in Taurus. The brown dwarf deficit revisited

Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs (BD) compared to the Trapezium cluster population (Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However, these works have concentrated on the highest stellar density regions of the Taurus cloud. We have performed a large scale optical survey of this region, covering a total area of 30 deg^2, and encompassing the densest part of the cloud as well as their surroundings, down to a mass detection limits of 15 Jupiter Masses (MJ). In this paper, we present the optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs. Combining our observations with previously published results, we derive an updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication that the relative numbers of BDs with respect to stars is decreased by a factor 2 in the central regions of the aggregates with respect to the more distributed population. Our findings are best explained in the context of the embryo-ejection model where brown dwarfs originate from dynamical interactions in small N unstable multiple systems.Comment: 20 pages, 15 figure

Accretion in the Rho-Oph pre-main sequence stars

The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region Rho-Oph. The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 Msun and it is complete to a limiting mass of ~0.05 Msun. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Pab or Brg. For comparison, we also obtained similar spectra of 35 diskless objects. The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (Macc ~ Mstar^1.8+-0.2) and a large spread, of two orders of magnitude at least, for any interval of Mstar. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than ~0.1Msun. The implications of these results are briefly discussed.Comment: A&A in press, 16 pages including tables, 5 figure

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=2.5=350 AU). The scattered light from this disk has undergone significant variability on a time scale 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_sun) relative to other nearby star-forming regions like IC 348, Chamaeleon I, and the Orion Nebula Cluster

The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry

We report on the properties of pre-main-sequence objects in the Taurus molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer Space Telescope. There are 215 previously-identified members of the Taurus star-forming region in our ~44 square degree map; these members exhibit a range of Spitzer colors that we take to define young stars still surrounded by circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no detectable dust excesses). We looked for new objects in the survey field with similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet imaging, and found 148 candidate new members of Taurus. We have obtained follow-up spectroscopy for about half the candidate sample, thus far confirming 34 new members, 3 probable new members, and 10 possible new members, an increase of 15-20% in Taurus members. Of the objects for which we have spectroscopy, 7 are now confirmed extragalactic objects, and one is a background Be star. The remaining 93 candidate objects await additional analysis and/or data to be confirmed or rejected as Taurus members. Most of the new members are Class II M stars and are located along the same cloud filaments as the previously-identified Taurus members. Among non-members with Spitzer colors similar to young, dusty stars are evolved Be stars, planetary nebulae, carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm

Spectroscopy of new brown dwarf members of rho Ophiuchi and an updated initial mass function

To investigate the universality hypothesis of the initial mass function in the substellar regime, the population of the rho Ophiuchi molecular cloud is analysed by including a new sample of low-mass spectroscopically confirmed members. To that end, we have conducted a large spectroscopic follow-up of young substellar candidates uncovered in our previous photometric survey. The spectral types and extinction were derived for a newly found population of substellar objects, and its masses estimated by comparison to evolutionary models. A thoroughly literature search was conducted to provide an up-to-date census of the cluster, which was then used to derive the luminosity and mass functions, as well as the ratio of brown dwarfs to stars in the cluster. These results were then compared to other young clusters. It is shown that the study of the substellar population of the rho Ophiuchi molecular cloud is hampered only by the high extinction in the cluster ruling out an apparent paucity of brown dwarfs. The discovery of 16 new members of rho Ophiuchi, 13 of them in the substellar regime, reveals the low-mass end of its population and shows the success of our photometric candidate selection with the WIRCam survey. The study of the brown dwarf population of the cluster reveals a high disk fraction of 76 (+5-8)%. Taking the characteristic peak mass of the derived mass function and the ratio of brown dwarfs to stars into account, we conclude that the mass function of rho Ophiuchi is similar to other nearby young clusters.Comment: Accepted to A&A (30 December 2011); v2 includes language editin

Four Brown Dwarfs in the Taurus Star-Forming Region

We have identified four brown dwarfs in the Taurus star-forming region. They were first selected from $R$ and $I$ CCD photometry of 2.29 square degrees obtained at the Canada-France-Hawaii Telescope. Subsequently, they were recovered in the 2MASS second incremental data release point source catalog. Low-resolution optical spectra obtained at the William Herschel telescope allow us to derive spectral types in the range M7--M9. One of the brown dwarfs has very strong H$\alpha$ emission (EW=-340 \AA). It also displays Br$\gamma$ emission in an infrared spectrum obtained with IRCS on the Subaru telescope, suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these Taurus objects are weaker than in field dwarfs of similar spectral type, consistent with low surface gravities as expected for young brown dwarfs. Two of the objects are cooler and fainter than GG Tau Bb, the lowest mass known member of the Taurus association. We estimate masses of only 0.03 M$_\odot$ for them. The spatial distribution of brown dwarfs in Taurus hints to a possible anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press

The Spitzer Infrared Spectrograph Survey of T Tauri Stars in Taurus

We present 161 Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars and young brown dwarfs in the Taurus star-forming region. All of the targets were selected based on their infrared excess and are therefore surrounded by protoplanetary disks; they form the complete sample of all available IRS spectra of T Tauri stars with infrared excesses in Taurus. We also present the IRS spectra of seven Class 0/I objects in Taurus to complete the sample of available IRS spectra of protostars in Taurus. We use spectral indices that are not significantly affected by extinction to distinguish between envelope-and disk-dominated objects. Together with data from the literature, we construct spectral energy distributions for all objects in our sample. With spectral indices derived from the IRS spectra we infer disk properties such as dust settling and the presence of inner disk holes and gaps. We find a transitional disk frequency, which is based on objects with unusually large 13-31 mu m spectral indices indicative of a wall surrounding an inner disk hole, of about 3%, and a frequency of about 20% for objects with unusually large 10 mu m features, which could indicate disk gaps. The shape and strength of the 10 mu m silicate emission feature suggests weaker 10 mu m emission and more processed dust for very low mass objects and brown dwarfs (spectral types M6-M9). These objects also display weaker infrared excess emission from their disks, but do not appear to have more settled disks than their higher-mass counterparts. We find no difference for the spectral indices and properties of the dust between single and multiple systems.NASANASA through JPL/CaltechNASA through the Spitzer Space TelescopeNational Science Foundation AST-0544588, 0901947Pennsylvania State UniversityEberly College of SciencePennsylvania Space Grant ConsortiumNSFAstronom