C. Briceño

The initial mass function in the Taurus star-forming region

By combining a deep optical imaging (I; z 0 ) survey of 8 deg 2 in the Taurus star-forming region with data from the Two-Micron All-Sky Survey (2MASS) and follow-up spectroscopy, we have performed a search for low-mass Taurus members that is complete to 0.02 Mfor reddenings of AV d4. We report the discovery of nine new members with spectral types of M5.75-M9.5, corresponding to masses of 0.1-0.015 Mby recent evolutionary models. The new M9.5 member is the least massive brown dwarf found to date in the Taurus star-forming region. We derive an initial mass function (IMF) for the fields surveyed in this work and in our previous studies, which encompass 54% of the known Taurus membership. We compare the Taurus IMF with a similarly derived one for the Trapezium Cluster and to mass functions for the M35 and Pleiades open clusters. While the IMFs in all of these regions flatten near � 0.8 M� , the mass function in Taurus is more nar- row and sharply peaked at this mass. Our survey indicates that Taurus has � 2 � fewer brown dwarfs at 0.02- 0.08 Mthan the Trapezium. We discuss the implications of these results for theories of the IMF, and suggest that the lower frequency of brown dwarfs in Taurus relative to the Trapezium may result from the low-den- sity star-forming environment, leading to larger minimum Jeans masses. Subject headings: infrared: stars — stars: evolution — stars: formation — stars: low-mass, brown dwarfs — stars: luminosity function, mass function — stars: pre-main-sequence

The Mass Accretion Rates of Intermediate-Mass T Tauri Stars

We present Hubble Space Telescope ultraviolet spectra and supporting ground-based data for a sample of nine intermediate-mass T Tauri stars (IMTTSs; 1.5–4 M⊙). The targets belong to three star-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZ Ori, P2441, and V1044 Ori in the Ori OB1c association surrounding the Orion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring around λ Ori. The supporting ground-based observations include nearly simultaneous UBV(R I)C photometry, 6 A resolution spectra covering the range 3900–7000 A, optical echelle observations in the range 5800–8600 A, and K-band near-infrared spectra. We use these data to determine improved spectral types and reddening corrections and to obtain physical parameters of the targets. We find that an extinction law with a weak 2175 A feature but high values of AUV/AV is required to explain the simultaneous optical-UV data; the reddening laws for two B-type stars located behind the Taurus clouds, HD 29647 and HD 283809, meet these properties. We argue that reddening laws with these characteristics may well be representative of cold, dense molecular clouds. Spectral energy distributions and emission-line profiles of the IMTTSs are consistent with expectations from magnetospheric accretion models. We compare our simultaneous optical-UV data with predictions from accretion shock models to get accretion luminosities and mass accretion rates () for the targets. We find that the average mass accretion rate for IMTTSs is ~3 × 10-8 M⊙ yr-1, a factor of ~5 higher than that for their low-mass counterparts. The new data extend the correlation between and stellar mass to the intermediate-mass range. Since the IMTTSs are evolutionary descendants of the Herbig Ae/Be stars, our results put limits to the mass accretion rates of their disks. We present luminosities of the UV lines of highly ionized metals and show that they are well above the saturation limit for magnetically active cool stars but correlate strongly with accretion luminosity, indicating that they are powered by accretion, in agreement with previous claims but using a sample in which reddening and accretion luminosities have been determined self-consistently. Finally, we find that the relation between accretion luminosity and Brγ luminosity found for low-mass T Tauri stars extends to the intermediate-mass regime.

Measuring accretion in young substellar objects : Approaching the planetary mass regime

We present observations of Hα emission-line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star-forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to ~0.015 M☉. Using models of Hα emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses 0.03 M☉, we find accretion rates of ~5 × 10-12 M☉ yr-1, among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub)stellar mass that we have noted previously for samples of more massive objects; the overall correlation is ∝ M2.1, and it now extends over a mass range of over 2 orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.

New low-mass members of the Taurus star-forming region

Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 deg2 of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have AV ≤ 8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M☉). Its spectrum contains extremely strong emission in Hα (Wλ ~ 950 A), as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong Hα are suggestive of intense accretion.

Herbig Ae/Be Stars in nearby OB Associations

We have carried out a study of the early-type stars in nearby OB associations spanning an age range of ~3–16 Myr, with the aim of determining the fraction of stars that belong to the Herbig Ae/Be class. We studied the B, A, and F stars in the nearby (≤500 pc) OB associations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1, with membership determined from Hipparcos data. We also included in our study the early-type stars in the Trumpler 37 cluster, part of the Cep OB2 association. We obtained spectra for 440 Hipparcos stars in these associations, from which we determined accurate spectral types, visual extinctions, effective temperatures, luminosities and masses, using Hipparcos photometry. Using colors corrected for reddening, we find that the Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearly different regions in the JHK diagram. Thus, we use the location on the JHK diagram, as well as the presence of emission lines and of strong 12 μm flux relative to the visual, to identify the Herbig Ae/Be stars in the associations. We find that the Herbig Ae/Be stars constitute a small fraction of the early-type stellar population even in the younger associations. Comparing the data from associations with different ages and assuming that the near-infrared excess in the Herbig Ae/Be stars arises from optically thick dusty inner disks, we determined the evolution of the inner disk frequency with age. We find that the inner disk frequency in the age range 3–10 Myr in intermediate-mass stars is lower than that in the low-mass stars (<1 M⊙); in particular, it is a factor of ~10 lower at ~3 Myr. This indicates that the timescales for disk evolution are much shorter in the intermediate-mass stars, which could be a consequence of more efficient mechanisms of inner disk dispersal (viscous evolution, dust growth, and settling toward the midplane).

Spitzer Observations of the Orion OB1 Association: Disk Census in the Low-Mass Stars

We present new Spitzer observations of two fields in the Orion OB1 association. We report 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: those with optically thick disks (class II systems), with an inner transitional disks (transitional disk candidates), and with evolved disks; the last exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify one transitional disk candidate in the 25 Orionis aggregate and three 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; larger degrees of dust settling are necessary to explain these objects.

The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 Kiloparsecs and Other Overdensities in the Outer Halo

We have measured the periods and light curves of 148 RR Lyrae variables from V = 13.5 to 19.7 from the first 100 deg2 of the Quasar Equatorial Survey Team RR Lyrae survey. Approximately 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital Sky Survey. According to our measurements, this feature has ~10 times the background density of halo stars, spans at least 375 by 35 in α and δ (≥30 by ≥3 kpc), lies ~50 kpc from the Sun, and has a depth along the line of sight of ~5 kpc (1 σ). These properties are consistent with the recent models that suggest that it is a tidal stream from the Sagittarius dwarf spheroidal galaxy. The mean period of the type ab variables, 0.58 days, is also consistent. In addition, we have found two smaller overdensities in the halo, one of which may be related to the globular cluster Pal 5.

Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr

We present the results of MMT observations of young stars for our study of protoplanetary disks at ages 1–10 Myr in two young clusters located in the Cepheus OB2 region: Trumpler 37 (embedded in the H II region IC 1396) and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying ~130 new members in Tr 37 and ~30 in NGC 7160. We use indicators of youth (Li absorption at 6707 A) and accretion/chromospheric activity (Hα emission) to identify and classify the low-mass cluster members. We derive spectral types for all the low-mass candidates and calculate the individual extinctions and the average over the clusters. With the extended member samples, we estimate the disk fraction in the clusters, finding that ~40% of the low-mass stars in Tr 37 are actively accreting, whereas only 1 of the ~55 low-mass stars in NGC 7160 shows indications of accretion. Optical photometry and theoretical isochrones are used to determine the age of the cluster members, confirming the estimates of ~4 Myr for Tr 37 and ~10 Myr for NGC 7160. Accretion rates in Tr 37 (~10-8 M⊙ yr-1 on average) are derived from U-band photometry. We find that only ~50% of the accreting stars have near-IR excesses (from 2MASS), which could be due to the geometry of their disks or be an indication dust of settling/grain growth. Finally, we study the high- and intermediate-mass members of the clusters. With the extended member list, we revise the spatial distribution of stars with disks. Our results are crucial for interpreting Spitzer Space Telescope studies of accretion disks at the ages of planet formation (3–10 Myr).

Spitzer Observations of the Orion OB1 Association: Second-Generation Dust Disks at 5-10 Myr

We report new Spitzer observations of intermediate-mass stars in two regions of the Orion OB1 association located in the subassociations OB1a (~10 Myr) and OB1b (~5 Myr). In a representative sample of stars earlier than F5 of both stellar groups, we find a population of stars surrounded of debris disks, without excess in the IRAC bands and without emission lines in their optical spectra, but with a varying degree of 24 ?m excess. Comparing our samples with 24 ?m observations of intermediate-mass stars in other stellar groups, spanning a range of ages from 2.5 to 150 Myr, we find that debris disks are more frequent and have larger 24 ?m excess at 10 Myr (OB1a). This trend agrees with predictions of models of evolution of solids in the outer regions of disks (>30 AU), where large icy objects (~1000 km) begin to form at ~10 Myr; the presence of these objects in the disk initiates a collisional cascade, producing enough dust particles to explain the relatively large 24 ?m excess observed in OB1a. The dust luminosity observed in the stellar groups older than 10 Myr declines roughly as predicted by collisional cascade models. Combining Spitzer observations, optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD 290543) and a star (HD 36444) with a large 24 ?m excess, both in OB1b. This last object could be explained as a intermediate stage between HAeBe and true debris systems or as a massive debris disk produced by a collision between two large objects (>1000 km).

The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b

We present results of a large-scale, multiepoch optical survey of the Orion OB1 association, carried out with the QUEST camera at the Venezuela National Astronomical Observatory. We identify for the first time the widely spread low-mass, young population in the Ori OB1a and OB1b subassociations. Candidate members were picked up by their variability in the V band and position in color-magnitude diagrams. We obtained spectra to confirm membership. In a region spanning ~68 deg2, we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 141 in Ori OB1b. The spatial distribution of the low-mass young stars is spatially coincident with that of the high-mass members but suggests a much sharper edge to the association. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori OB1b is concentrated within a ringlike structure of radius ~2°(~15 pc at 440 pc), centered roughly on the star Ori in the Orion belt. The ring is apparent in 13CO and corresponds to a region with an extinction AV ≥ 1. The stars exhibiting strong Hα emission, an indicator of active accretion, are found along this ring, whereas the center is populated with weak Hα-emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars within a ~3 deg2 area located in Ori OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Be star V346 Ori is also associated with this grouping, which could be an older analog of σ Ori. Using several sets of evolutionary tracks, we find an age of 7–10 Myr for Ori OB1a and of ~4–6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Hα and near-IR excesses show that the number of accreting low-mass stars decreases sharply between Ori OB1b and Ori OB1a. These results indicate that although a substantial fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially dissipated by 10 Myr.