Showing papers by "Lee Hartmann published in 2003"

Accretion in Young Stellar/Substellar Objects

Abstract: We present a study of accretion in a sample of 45 young, low-mass objects in a variety of star-forming regions and young associations, about half of which are likely substellar. Based primarily on the presence of broad, asymmetric Hα emission, we have identified 13 objects (~30% of our sample) that are strong candidates for ongoing accretion. At least three of these are substellar. We do not detect significant continuum veiling in most of the accretors with late spectral types (M5-M7). Accretion shock models show that lack of measurable veiling allows us to place an upper limit to the mass accretion rates of 10-10 M☉ yr-1. Using magnetospheric accretion models with appropriate (sub)stellar parameters, we can successfully explain the accretor Hα emission-line profiles and derive quantitative estimates of accretion rates in the range 10-12 M☉ yr-1 < < 10-9 M☉ yr-1. There is a clear trend of decreasing accretion rate with stellar mass, with mean accretion rates declining by 3-4 orders of magnitude over ~1-0.05 M☉.

Unveiling the Inner Disk Structure of T Tauri Stars

Abstract: We present near-infrared spectra of the excess continuum emission from the innermost regions of classical T Tauri disks. In almost all cases, the shape of the excess is consistent with that of a single-temperature blackbody with T ~ 1400 K, similar to the expected dust sublimation temperature for typical dust compositions. The amount of excess flux roughly correlates with the accretion luminosity in objects with similar stellar properties. We compare our observations with the predictions of simple disk models having an inner rim located at the dust sublimation radius, including irradiation heating of the dust from both the stellar and accretion luminosities. The models yield inner rim radii in the range 0.07-0.54 AU, increasing with higher stellar and accretion luminosities. Using typical parameters that fit our observed sample, we predict a rim radius ~0.2 AU for the T Tauri star DG Tau, which agrees with recent Keck near-infrared interferometric measurements. For large mass accretion rates, the inner rim lies beyond the corotation radius at (or within) which magnetospheric accretion flows are launched, which implies that pure gaseous disks must extend inside the dust rim. Thus, for a significant fraction of young stars, dust cannot exist in the innermost disk, calling into question theories in which solid particles are ejected by a wind originating at the magnetospheric radius.

New Low-Mass Members of the Taurus Star-forming Region

Abstract: 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 M275 through M9 The new M9 source has the second latest spectral type of the known members of Taurus (~002 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

Comments on Inferences of Star Formation Histories and Birth Lines

Abstract: Palla & Stahler have recently argued that star formation in Taurus and other nearby molecular clouds extends over a period of at least 10 Myr, implying quasi-static cloud evolution and star formation. Their conclusions contradict other recent results indicating that molecular clouds are transient objects and star formation proceeds rapidly. The Palla & Stahler picture implies that most molecular clouds should have extremely low rates of star formation and that in such inactive stages, the stellar initial mass function (IMF) should be strongly skewed toward producing stars with masses 1 M☉; neither prediction is supported by observations. I show that the Palla & Stahler conclusions for Taurus depend almost entirely on a small number of stars with masses 1 M☉; the lower mass stars show no evidence for such an extended period of star formation. I further show that most of the stars apparently older than 10 Myr in the direction of Taurus are probably foreground nonmembers. I also present birth line calculations that support the idea that the ages of the stars with masses 1 M☉ have been systematically overestimated because birth line age corrections have been underestimated; such birth lines would eliminate the need to postulate skewed IMFs. The simplest and most robust explanation of current observations characterizing the vast majority of young stars in molecular clouds is that cloud and star formation is rapid and dynamic.

Viscous diffusion and photoevaporation of stellar disks

Abstract: The evolution of a stellar disk under the influence of viscous evolution, photoevaporation from the central source, and photoevaporation by external stars is studied. We take the typical parameters of T Tauri stars (TTSs) and the Trapezium Cluster conditions. The photoionizing flux from the central source is assumed to arise from both the quiescent star and accretion shocks at the base of stellar magnetospheric columns, along which material from the disk accretes. The accretion flux is calculated self-consistently from the accretion mass-loss rate. We find that the disk cannot be entirely removed using only viscous evolution and photoionization from the disk-star accretion shock. However, when FUV photoevaporation by external massive stars is included, the disk is removed in 106-107 yr, and when EUV photoevaporation by external massive stars is included, the disk is removed in 105-106 yr. An intriguing feature of photoevaporation by the central star is the formation of a gap in the disk at late stages of the disk evolution. As the gap starts forming, viscous spreading and photoevaporation work in resonance. When viscous accretion and photoevaporation by the central star and external massive stars are considered, the disk shrinks and is truncated at the gravitational radius, where it is quickly removed by the combination of viscous accretion, viscous spreading, photoevaporation from the central source, and photoevaporation by the external stars. There is no gap formation for disks nearby external massive stars because the outer annuli are quickly removed by the dominant EUV flux. On the other hand, at larger, more typical distances (d 0.03 pc) from the external stars the flux is FUV-dominated. As a consequence, the disk is efficiently evaporated at two different locations, forming a gap during the last stages of the disk evolution.

Why are the K dwarfs in the Pleiades so Blue

Abstract: The K dwarfs in the Pleiades fall nearly 1 mag below a main-sequence isochrone when plotted in a color- magnitude diagram utilizing V magnitude as the luminosity index and BV as the color index. This peculiarity has been known for 40 years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using BV as the color index. However, using VI as the color index, stars in the two clusters are coincident to MV � 10; using VK as the color index, Pleiades late K and M stars fall above the main-sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (� 50%). Weak- lined T Tauri stars have similar color anomalies, and we suspect that this is a common feature of all very young K dwarfs (spectral type >K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre-main-sequence isochrone-fitting ages for clusters like the Pleiades, since the age derived will depend on the temperature index used.

A comprehensive study of the l1551 irs 5 binary system

Abstract: We model the Class I source L1551 IRS 5, adopting a flattened infalling envelope surrounding a binary disk system and a circumbinary disk. With our composite model, we calculate self-consistently the spectral energy distribution of each component of the L1551 IRS 5 system, using additional constraints from recent observations by ISO, the water ice feature from observations with SpeX, the SCUBA extended spatial brightness distribution at submillimeter wavelengths, and the VLA spatial intensity distributions at 7 mm of the binary disks. We analyze the sensitivity of our results to the various parameters involved. Our results show that a flattened-envelope collapse model is required to explain simultaneously the large-scale fluxes and the water ice and silicate features. On the other hand, we find that the circumstellar disks are optically thick in the millimeter range and are inclined so that their outer parts hide the emission along the line of sight from their inner parts. We also find that these disks have lower mass accretion rates than the infall rate of the envelope.

The Spatial Distribution of Fluorescent H$_2$ Near T Tau

Abstract: New subarcsecond FUV observations of T Tau with HST/STIS show spatially resolved structures in the 2"x2" area around the star. The structures show in multiline emission of fluorescent H_2 pumped by Lyman alpha. One emission structure follows the cavity walls observed around T Tau N in scattered light in the optical. A temperature greater or equal to 1000K is required to have enough population in the H_2 to produce the observed fluorescent lines; in the cool environment of the T Tau system, shock heating is required to achieve this temperature at distances of a few tens of AU. Fluorescent H_2 along the cavity wall represents the best evidence to date for the action of low-density, wide-opening-angle outflows driving cavities into the molecular medium at scales smaller than 100 AU. A southern region of emission consists of two arcs, with shape and orientation similar to the arcs of H_2 2.12 microns and forbidden line emission crossing the outflow associated with the embedded system T Tau S. This region is located near the centroid of forbidden line emission at the blueshifted lobe of the N-S outflow.

The Spatial Distribution of Fluorescent H2 Emission near T Tauri

Abstract: New subarcsecond far-UV observations of T Tau with Hubble Space Telescope STIS show spatially resolved structure in the 2in. x 2in. area around the star. The structures are apparent in multiline emission of fluorescent H2 pumped by Ly(alpha). One emission structure follows the cavity walls observed around T Tau N in scattered light in the optical. A temperature of greater than or = l000 K is required to have a high enough population in the H2 to produce the observed fluorescent lines; in the cool environment of the T Tau system, shock heating is required to achieve this temperature at distances of a few tens of AU. Fluorescent H2 along the cavity wall represent the best evidence to date for the action of low-density, wide opening angle outflows driving cavities into the molecular medium at scales less than or = 100 AU. A southern region of emission consists of two arcs, with shape and orientation similar to the arcs of H2 2.12 microns and forbidden-line emission crossing the outflow associated with the embedded system T Tau S. This region is located near the centroid of forbidden-line emission at the blueshifted lobe of the north-south outflow.

Stellar Outflows Driven by Magnetized Wide-Angle Winds

Abstract: We present two-dimensional, cylindrically symmetric simulations of hydrodynamic and magnetohydrodynamic (MHD) wide-angle winds interacting with a collapsing environment. These simulations have direct relevance to young stellar objects. The results may also be of use in the study of collimated outflows from proto-planetary and planetary nebulae. We study a range of wind configurations consistent with asymptotic MHD wind collimation. The degree of collimation is parameterized by the ratio of the wind density at the pole to that of the equator. We find that a toroidal magnetic field can have a significant influence on the resulting outflow, giving rise to a very dense, jetlike flow in the postshock region. The properties of the flow in this region are similar to the asymptotic state of a collimated MHD wind. We conclude that wide-angle MHD winds are quite likely capable of driving molecular outflows. Because of difficulty in treating MHD winds ab initio in simulations, we choose magnetic field strengths in the wind consistent with slow magnetic rotators. While MHD-launched winds will be in the fast rotator regime, we discuss how our results, which rely on toroidal pinch effects, will hold for stronger field strengths.

Accretion and Ejection

Abstract: The processes by which stars stars accrete their mass are not fully understood. Accretion in early stellar evolution appears to be highly timevariable. Since accretion and mass ejection are closely connected — the energy for winds and jets comes from accretion — this means that mass loss is also highly time-variable. Understanding the origin of this variable accretion is an important outstanding issue in star formation that has not received su cient attention.

Accretion in Young Stellar/Substellar Objects

Abstract: We present a study of accretion in a sample of 45 young, low mass objects in a variety of star forming regions and young associations, about half of which are likely substellar. Based primarily on the presence of broad, asymmetric Halpha emission, we have identified 13 objects (~30% of our sample) which are strong candidates for ongoing accretion. At least 3 of these are substellar. We do not detect significant continuum veiling in most of the accretors with late spectral types (M5-M7). Accretion shock models show that lack of measurable veiling allows us to place an upper limit to the mass accretion rates of <~ 10^{-10} Msun/yr. Using magnetospheric accretion models with appropriate (sub)stellar parameters, we can successfully explain the accretor Halpha emission line profiles, and derive quantitative estimates of accretion rates in the range 10^{-12} < Mdot < 10^{-9} Msun/yr. There is a clear trend of decreasing accretion rate with stellar mass, with mean accretion rates declining by 3-4 orders of magnitude over ~ 1 - 0.05 Msun.

Unveiling the Inner Disk Structure of T Tauri Stars

Abstract: We present near-infrared spectra of the excess continuum emission from the innermost regions of classical T Tauri disks. In almost all cases, the shape of the excess is consistent with that of a single-temperature blackbody with T ~ 1400 K, similar to the expected dust sublimation temperature for typical dust compositions. The amount of excess flux roughly correlates with the accretion luminosity in objects with similar stellar properties. We compare our observations with the predictions of simple disk models having an inner rim located at the dust sublimation radius, including irradiation heating of the dust from both the stellar and accretion luminosities. The models yield inner rim radii in the range 0.07-0.54 AU, increasing with higher stellar and accretion luminosities. Using typical parameters which fit our observed sample, we predict a rim radius ~ 0.2 AU for the T Tauri star DG Tau, which agrees with recent Keck near-infrared interferometric measurements. For large mass accretion rates, the inner rim lies beyond the corotation radius at (or within) which magnetospheric accretion flows are launched, which implies that pure gaseous disks must extend inside the dust rim. Thus, for a significant fraction of young stars, dust cannot exist in the innermost disk, calling into question theories in which solid particles are ejected by a wind originating at the magnetospheric radius.

The Brown Dwarf Deficit in Taurus: Evidence for a Non-Universal IMF

Abstract: We present the results of a deep, optical/IR wide field imaging survey of selected fields in the nearby (d ~ 140 pc) Taurus star-forming region. We report the discovery of 9 new members with spectral types M5.75-M9.5. We derive an Initial Mass Function encompassing 54% of the known members in Taurus. Comparison with dense regions like the Trapezium Cluster in Orion shows that Taurus has produced x2 less brown dwarfs. We suggest that the lower frequency of brown dwarfs in Taurus may result from the low-density star-forming environment, leading to larger minimum Jeans masses.

Models of Accretion Disks around Young Stars

Abstract: We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.

Models of Accretion Disks around Young Stars

Abstract: We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.

Testing the Paradigm of Low-Mass Star Formation

Abstract: Protostellar core formation is probably much more dynamic, and magnetic fields are probably much less important, than has been previously assumed in the standard model of low-mass star formation. This revised picture has important consequences: it is easier to understand the observed rapidity of star formation in molecular clouds; cores are more likely to have structures favoring high infall rates at early times, helping to explain the differences between Class 0 and Class I protostars; and core structure and asymmetry will strongly favor post-collapse fragmentation into binary and multiple stellar systems.