Publications (16)29.54 Total impact
-
Article: Evidence for Environmental Dependence of the Upper Stellar Initial Mass Function in Orion A
[show abstract] [hide abstract]
ABSTRACT: We extend our previous study of the stellar population of L1641, the lower-density star-forming region of the Orion A cloud south of the dense Orion Nebula Cluster (ONC), with the goal of testing whether there is a statistically significant deficiency of high-mass stars in low-density regions. Previously, we compared the observed ratio of low-mass stars to high-mass stars with theoretical models of the stellar initial mass function (IMF) to infer a deficiency of the highest-mass stars in L1641. We expand our population study to identify the intermediate mass (late B to G) L1641 members in an attempt to make a more direct comparison with the mass function of the nearby ONC. The spectral type distribution and the K-band luminosity function of L1641 are similar to those of the ONC (Hillenbrand 1997; Muench et al. 2002), but problems of incompleteness and contamination prevent us from making a detailed test for differences. We limit our analysis to statistical tests of the ratio of high-mass to low-mass stars, which indicate a probability of only 3% that the ONC and the southern region of L1641 were drawn from the same population, supporting the hypothesis that the upper mass end of the IMF is dependent on environmental density.12/2012; -
Article: Evolution of X-ray and FUV Disk-Dispersing Radiation Fields
[show abstract] [hide abstract]
ABSTRACT: We present new X-ray and Far Ultraviolet (FUV) observations of T Tauri stars covering the age range 1 to 10 Myr. Our goals are to observationally constrain the intensity of radiation fields responsible for evaporating gas from the circumstellar disk and to assess the feasibility of current photoevaporation models, focusing on X-ray and UV radiation. We greatly increase the number of 7-10 Myr old T Tauri stars observed in the X-rays by including observations of the well populated 25 Ori aggregate in the Orion OB1a subassociation. With these new 7-10 Myr objects, we confirm that X-ray emission remains constant from 1-10 Myr. We also show, for the first time, observational evidence for the evolution of FUV radiation fields with a sample of 56 accreting and non-accreting young stars spanning 1 Myr to 1 Gyr. We find that the FUV emission decreases on timescales consistent with the decline of accretion in classical T Tauri stars until reaching the chromospheric level in weak T Tauri stars and debris disks. Overall, we find that the observed strength of high energy radiation is consistent with that required by photoevaporation models to dissipate the disks in timescales of approximately 10 Myr. Finally, we find that the high energy fields that affect gas evolution are not similarly affecting dust evolution; in particular, we find that disks with inner clearings, the transitional disks, have similar levels of FUV emission as full disks.02/2011; -
Article: Spitzer Observations of the Lambda Orionis cluster. II. Disks around solar-type and low mass stars
[show abstract] [hide abstract]
ABSTRACT: We present IRAC/MIPS Spitzer Space Telescope observations of the solar type and the low mass stellar population of the young (~5 Myr) Lambda Orionis cluster. Combining optical and 2MASS photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 solar mass to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution (SED) slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with ``evolved disks''( with smaller excesses than optically thick disk systems), and ``transitional disks'' candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K type stars (Rc-J<2) to ~27% for stars with spectral type M5 or later (Rc-J>4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the Lambda Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5 Myr. Comment: 37 pages, 11 Figures and 3 Tables, Accepted for publication in The Astrophysical Journal08/2010; -
Article: The Stellar Population of h and χ Persei: Cluster Properties, Membership, and the Intrinsic Colors and Temperatures of Stars
[show abstract] [hide abstract]
ABSTRACT: From photometric observations of ~ 47,000 stars and spectroscopy of ~ 11,000 stars, we describe the first extensive study of the stellar population of the famous Double Cluster, h and χ Persei, down to subsolar masses. By analyzing optical spectra and optical/infrared photometry, we constrain the distance moduli (dM), reddening (E(B – V)), and ages for h Persei, χ Persei, and the low-density halo population surrounding both cluster cores. With the exception of mass and spatial distribution, the clusters are nearly identical in every measurable way. Both clusters have E(B – V) ~ 0.52-0.55 and dM = 11.8-11.85; the halo population, while more poorly constrained, likely has identical properties. As determined from the main-sequence turnoff, the luminosity of M supergiants, and pre-main-sequence isochrones, ages for h Persei, χ Persei, and the halo population all converge on 14 Myr, thus showing a stunning agreement between estimates based on entirely different physics. From these data, we establish the first spectroscopic and photometric membership lists of cluster stars down to early/mid M dwarfs. At minimum, there are ~ 5000 members within 10' of the cluster centers, while the entire h and χ Persei region has at least ~ 13,000 and as many as 20,000 members. The Double Cluster contains 8400 M ☉ of stars within 10' of the cluster centers. We estimate a total mass of at least 20,000 M ☉. We conclude our study by outlining outstanding questions regarding the past and present properties of h and χ Persei. From comparing recent work, we compile a list of intrinsic colors and derive a new effective temperature scale for O-M dwarfs, giants, and supergiants.The Astrophysical Journal Supplement Series 01/2010; 186(2):191. · 13.46 Impact Factor -
Article: The Stellar Population of h and chi Persei: Cluster Properties, Membership, and the Intrinsic Colors and Temperatures of Stars
[show abstract] [hide abstract]
ABSTRACT: (Abridged) From photometric observations of $\sim$ 47,000 stars and spectroscopy of $\sim$ 11,000 stars, we describe the first extensive study of the stellar population of the famous Double Cluster, h and $\chi$ Persei, down to subsolar masses. Both clusters have E(B-V) $\sim$ 0.52--0.55 and dM = 11.8--11.85; the halo population, while more poorly constrained, likely has identical properties. As determined from the main sequence turnoff, the luminosity of M supergiants, and pre-main sequence isochrones, ages for h Persei, $\chi$ Persei and the halo population all converge on $\approx$ 14 Myr. From these data, we establish the first spectroscopic and photometric membership lists of cluster stars down to early/mid M dwarfs. At minimum, there are $\sim$ 5,000 members within 10' of the cluster centers, while the entire h and $\chi$ Persei region has at least $\sim$ 13,000 and as many as 20,000 members. The Double Cluster contains $\approx$ 8,400 M$_{\odot}$ of stars within 10' of the cluster centers. We estimate a total mass of at least 20,000 M$_{\odot}$. We conclude our study by outlining outstanding questions regarding the properties of h and $\chi$ Persei. From comparing recent work, we compile a list of intrinsic colors and derive a new effective temperature scale for O--M dwarfs, giants, and supergiants. Comment: 88 pages, many figures, Accepted for publication in The Astrophysical Journal Supplements. Contact lead author for version with high-resolution figures11/2009; -
Article: The Differential Rotation of FU Ori
[show abstract] [hide abstract]
ABSTRACT: The emission of FU Orionis objects in outburst has been identified as arising in rapidly accreting protoplanetary disks, based on a number of observational properties. A fundamental test of the accretion disk scenario is that the differentially rotating disk spectrum should produce a variation of rotational velocity with the wavelength of observation, as spectra taken at longer wavelengths probe outer, more slowly rotating disk regions. Previous observations of FU Ori have shown smaller rotation at near-infrared (~2.2 μm) wavelengths than observed at optical (~0.6 μm) wavelengths consistent with the assumption of Keplerian rotation. Here we report a spectrum from the Phoenix instrument on Gemini South which shows that differential (slower) rotation continues to be observed out to ~5 μm. The observed spectrum is well matched by the prediction of our accretion disk model previously constructed to match the observed spectral energy distribution and the differential rotation at wavelengths 2.2 μm. This kinematic result allows us to confirm our previous inference of a large outer radius (~1 AU) for the rapidly accreting region of the FU Ori disk, which presents difficulties for outburst models relying purely on thermal instability. While some optical spectra have been interpreted to pose problems for the disk interpretation of FU Ori, we show that the adjustment of the maximum effective temperature of the disk model, proposed in a previous paper, greatly reduces these difficulties.The Astrophysical Journal 02/2009; 694(1):L64. · 6.02 Impact Factor -
Article: The X-Ray Environment During the Epoch of Terrestrial Planet Formation: Chandra Observations of h Persei
[show abstract] [hide abstract]
ABSTRACT: We describe Chandra/ACIS-I observations of the massive ~ 13-14 Myr-old cluster, h Persei, part of the famous Double Cluster (h and χ Persei) in Perseus. Combining the list of Chandra-detected sources with new optical/IR photometry and optical spectroscopy reveals ~ 165 X-ray bright stars with V 23. Roughly 142 have optical magnitudes and colors consistent with cluster membership. The observed distribution of Lx peaks at Lx ~ 1030.3 erg s–1 and likely traces the bright edge of a far larger population of 0.4-2 M ☉ X-ray active stars. From a short list of X-ray active stars with IRAC 8 μm excess from warm, terrestrial zone dust, we derive a maximum X-ray flux incident on forming terrestrial planets. Although there is no correlation between X-ray activity and IRAC excess, the fractional X-ray luminosity correlates with optical colors and spectral type. By comparing the distribution of Lx /L versus spectral type and V – I in h Per with results for other 1-100 Myr-old clusters, we show that stars slightly more massive than the Sun ( 1.5 M ☉) fall out of X-ray saturation by 10-15 Myr. Changes in stellar structure for 1.5 M ☉ stars likely play an important role in this decline of X-ray emission.The Astronomical Journal 01/2009; 137(2):3210. · 4.03 Impact Factor -
Article: The Hot Inner Disk of FU Orionis
[show abstract] [hide abstract]
ABSTRACT: We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~4000 Å to ~8 μm. Using an estimated visual extinction AV ~ 1.5, a steady disk model with a central star mass ~0.3 M☉, and a mass accretion rate ~2 × 10-4 M☉ yr-1, we can reproduce the SED of FU Ori quite well. Higher values of extinction used in previous analysis (AV ~ 2.1) result in SEDs which are less well fitted by a steady disk model, but might be explained by extra energy dissipation of the boundary layer in the inner disk. With the mid-infrared spectrum obtained by the IRS on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~1 AU, using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc ≥ 1000 K out to ~0.5-1 AU, suggesting that the magnetorotational instability can be supported out to that distance. Assuming that the ~100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter to be α ~ 0.2-0.02 in the outburst state, consistent with numerical simulations of the magnetorotational instability in disks. The radial extent of the high- region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational and magnetorotational instabilities.The Astrophysical Journal 12/2008; 669(1):483. · 6.02 Impact Factor -
Article: The X-Ray Environment During the Epoch of Terrestrial Planet Formation: Chandra Observations of h Persei
[show abstract] [hide abstract]
ABSTRACT: We describe Chandra/ACIS-I observations of the massive ~ 13--14 Myr-old cluster, h Persei, part of the famous Double Cluster (h and chi Persei) in Perseus. Combining the list of Chandra-detected sources with new optical/IR photometry and optical spectroscopy reveals ~ 165 X-ray bright stars with V < 23. Roughly 142 have optical magnitudes and colors consistent with cluster membership. The observed distribution of Lx peaks at Lx ~ 10^30.3 ergs/s and likely traces the bright edge of a far larger population of ~ 0.4--2 Msun X-ray active stars. From a short list of X-ray active stars with IRAC 8 micron excess from warm, terrestrial-zone dust, we derive a maximum X-ray flux incident on forming terrestrial planets. Although there is no correlation between X-ray activity and IRAC excess, the fractional X-ray luminosity correlates with optical colors and spectral type. By comparing the distribution of Lx/L* vs. spectral type and V-I in h Per with results for other 1--100 Myr-old clusters, we show that stars slightly more massive than the Sun (> 1.5 Msun) fall out of X-ray saturation by ~ 10--15 Myr. Changes in stellar structure for > 1.5 Msun stars likely play an important role in this decline of X-ray emission. Comment: 34 pages, 7 Figures, 2 Tables; Accepted for publication in The Astronomical Journal11/2008; -
Article: Long-wavelength excesses of FU Orionis objects: flared outer disks or infalling envelopes?
[show abstract] [hide abstract]
ABSTRACT: The mid- to far-infrared emission of the outbursting FU Orionis objects has been attributed either to a flared outer disk or to an infalling envelope. We revisit this issue using detailed radiative transfer calculations to model the recent, high signal-to-noise data from the IRS instrument on the {Spitzer Space Telescope}. In the case of FU Ori, we find that a physically-plausible flared disk irradiated by the central accretion disk matches the observations. Building on our previous work, our accretion disk model with outer disk irradiation by the inner disk reproduces the spectral energy distribution between ~4000 angstroms to ~40 microns. Our model is consistent with near-infrared interferometry but there are some inconsistencies with mid-infared interferometric results. Including the outer disk allows us to refine our estimate of the outer radius of the outbursting, high mass accretion rate disk in FU Ori as ~ 0.5 AU, which is a crucial parameter in assessing theories of the FU Orionis phenomenon. We are able to place an upper limit on the mass infall rate of any remnant envelope infall rate to ~ 7e-7 Msun/yr assuming a centrifugal radius of 200 AU. The FUor BBW 76 is also well modelled by a 0.6 AU inner disk and a flared outer disk. However, V1515 Cyg requires an envelope with an outflow cavity to adequately reproduce the IRS spectrum. In contrast with the suggestion by Green et al., we do not require a flattened envelope to match the observations; the inferred cavity shape is qualitatively consistent with typical protostellar envelopes. This variety of dusty structures suggests that the FU Orionis phase can be present at either early or late stages of protostellar evolution. Comment: 30 pages, 9 figures, Accepted by ApJ06/2008; -
Article: A Spitzer view of protoplanetary disks in the gamma Velorum cluster
[show abstract] [hide abstract]
ABSTRACT: We present new Spitzer Space Telescope observations of stars in the young ~5 Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified 5 debris disks around A-type stars, and 5-6 debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the IR flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that gamma Velorum stellar cluster is slightly older than reported ages and the the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr. Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11 Figures06/2008; -
Article: Spitzer observations of the Orion OB1 association: disk census in the low mass stars
[show abstract] [hide abstract]
ABSTRACT: 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 Journal09/2007; -
Article: The Hot Inner Disk of FU Ori
[show abstract] [hide abstract]
ABSTRACT: We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~ 4000 angstrom, to ~ 8 micron. Using an estimated visual extinction Av~1.5, a steady disk model with a central star mass ~0.3 Msun and a mass accretion rate ~ 2e-4 Msun/yr, we can reproduce the spectral energy distribution of FU Ori quite well. With the mid-infrared spectrum obtained by the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~ 1 AU using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~ 0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc > 1000 K out to ~ 0.5 - 1 AU, suggesting that the magnetorotational instability (MRI) can be supported out to that distance. Assuming that the ~ 100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter (alpha) to be ~ 0.2 - 0.02 in the outburst state, consistent with numerical simulations of MRI in disks. The radial extent of the high mass accretion region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational instability and MRI. Comment: 32 pages, 10 figures, to appear in the Astrophysical Journal07/2007; -
Article: Spitzer Space Telescope study of disks in the young $\sigma$ Orionis cluster
[show abstract] [hide abstract]
ABSTRACT: We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks(like classical T Tauri stars), class I (protostellar) candidates, and stars with ``evolved disks''; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC infrared excesses found in stellar clusters and associations with and without central high mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.01/2007; -
Article: Disk evolution in the Ori OB1 association
[show abstract] [hide abstract]
ABSTRACT: We analyze multi-band photometry of a subsample of low mass stars in the associations Ori OB1a and 1b discovered during the CIDA Orion Variability Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26 Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b. We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and 18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected at 10um while only one was detected at 18um. We estimate mass accretion rates from the excess luminosity at U, and find that they are consistent with determinations for a number of other associations, with or without high mass star formation. The observed decrease of mass accretion rate with age is qualitatively consistent with predictions of viscous evolution of accretion disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to Ori OB1a. This decrease implies that significant grain growth and settling towards the midplane has taken place in the inner disks of Ori OB1. We compare the SED of the star detected at both 10um and 18um with disk models for similar stellar and accretion parameters. We find that the low <= 18 um fluxes of this Ori OB1b star cannot be due to the smaller disk radius expected from viscous evolution in the presence of the FUV radiation fields from the OB stars in the association. Instead, we find that the disk of this star is essentially a flat disk, with little if any flaring, indicating a a significant degree of dust settling towards the midplane, as expected from dust evolution in protoplanetary disks.11/2004; -
Article: The CIDA Variability Survey of Orion OB1. I: the low-mass population of Ori OB 1a and 1b
[show abstract] [hide abstract]
ABSTRACT: We present results of a large scale, multi-epoch optical survey of the Ori 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 Orion OB1a and OB1b sub-associations. 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 deg^2 we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori 1b is concentrated within a ring-like structure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the star epsilon 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 Ha emission, an indicator of active accretion, are found along this ring, while the center is populated with weak Ha 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 deg^2 area located in 1a, 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 sigma Ori. Using using several sets of evolutionary tracks we find an age of 7 - 10 Myr for Ori 1a and of ~4 - 6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Ha and near-IR excesses show that while a substantial fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially dissipated by 10 Myr. Comment: 44 pages, 12 figures, to appear in the Astronomical Journal. (Abridged abstract - to fit length limit in astroph) Full resolution figures in http://www.cida.ve/~briceno/publications/10/2004;
Top co-authors
Top Journals
Institutions
-
2010
-
Centro de Investigaciones de Astronomia (CIDA)
Mérida, Estado Merida, Venezuela
-
-
2007–2009
-
University of Michigan
- Department of Astronomy
Ann Arbor, MI, USA
-
-
2004
-
Idenix Pharmaceuticals, Inc.
Cambridge, MA, USA
-
