948 research outputs found
Accretion in Brown Dwarfs: an Infrared View
This paper presents a study of the accretion properties of 19 very low mass
objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8
objects we obtained high resolution Halpha profiles and determined mass
accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in
emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using
objects for which we have both a determination of Lacc from Halpha and a Pabeta
detection, we show that the correlation between the Pabeta luminosity and
luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus,
extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure
Lacc also in the substellar regime. The results were less conclusive for
Brgamma, which was detected only in 2 objects, neither of which had an Halpha
estimate of Macc. Using the relation between L(Pab) and Lacc we determined the
accretion rate for all the objects in our sample (including those with no
Halpha spectrum), more than doubling the number of substellar objects with
known Macc. When plotted as a function of the mass of the central object
together with data from the literature, our results confirm the trend of lower
Macc for lower Mstar, although with a large spread. Some of the spread is
probably due to an age effect; our very young objects in rho-Oph have on
average an accretion rate at least one order of magnitude higher than objects
of similar mass in older regions. As a side product, we found that the width of
Halpha measured at 10% peak intensity is not only a qualitative indicator of
accretion, but can be used to obtain a quantitative estimate of Macc over a
large mass range, from T Tauri stars to brown dwarfs. Finally, we found that
some of our objects show evidence of mass-loss.Comment: 11 pages, 8 figures, A&A in pres
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
The 2008 extreme outburst of the young eruptive variable star Ex Lupi
In early 2008, the young low-mass star EX Lupi, the prototype of the EXor
class of eruptive variables, optically brightened by over five magnitudes for a
period of 7 months. The previous time a change of such amplitude had been
observed in EX Lup was over 50 years ago. In this Letter we present new optical
and near-IR high resolution spectroscopy of EX~Lup during the 2008 outburst. We
investigate the physical characteristics of the outburst both soon after it
began and some four months later, and consider the energetics and kinematics
observed. Emission line strengths, widths, and profiles changed significantly
between the two observations. Also, modeling of the 2.2935 um CO overtone
bandhead emission suggests that an inner gap in the circumstellar gas disk
around the star may be present and it is from the inner edge of the gas disk
that the CO overtone emission probably arises. We derive a mass accretion
luminosity and rate during the extreme outburst of ~2+-0.5~Lsun and
~2+-0.5x10^-7 Msun yr^-1, respectively, which suggests that this outburst was
indeed one of the strongest witnessed in EX Lup, yet not as strong as those
observed in FU Orionis stars.Comment: 5 pages, 4 figure
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
Photometric determination of the mass accretion rates of pre-main sequence stars. I. Method and application to the SN1987A field
We have developed and successfully tested a new self-consistent method to
reliably identify pre-main sequence (PMS) objects actively undergoing mass
accretion in a resolved stellar population, regardless of their age. The method
does not require spectroscopy and combines broad-band V and I photometry with
narrow-band Halpha imaging to: (1) identify all stars with excess Halpha
emission; (2) derive their Halpha luminosity L(Halpha); (3) estimate the Halpha
emission equivalent width; (4) derive the accretion luminosity L_acc from
L(Halpha); and finally (5) obtain the mass accretion rate M_acc from L_acc and
the stellar parameters (mass and radius). By selecting stars with photometric
accuracy in Halpha better than 15%, the statistical uncertainty on the derived
M_acc is typically <17% and is dictated by the precision of the Halpha
photometry. Systematic uncertainties, of up to a factor of 3 on the value of
M_acc, are caused by our incomplete understanding of the physics of the
accretion process and affect all determinations of the mass accretion rate,
including those based on a spectroscopic Halpha line analysis.
As an application of our method, we study a field of 9.16 arcmin2 around
SN1987A, using existing HST photometry. We identify as bona-fide PMS stars a
total of 133 objects with a Halpha excess above the 4 sigma level and a median
age of 13.5 Myr. Their median mass accretion rate of 2.6x10-8 Msolar/yr is in
excellent agreement with previous determinations based on the U-band excess of
the stars in this field, and with the value measured for G-type PMS stars in
the Milky Way. Their L_acc shows a strong dependence on their distance from a
group of hot massive stars in the field and suggests that the UV radiation of
the latter is rapidly eroding the circumstellar discs around PMS stars.Comment: 18 pages, 18 figures, accepted for publication in the Astrophysical
Journa
Evidence for accretion in a nearby, young brown dwarf
We report on the discovery of the young, nearby, brown dwarf 2MASS
J0041353562112. The object has a spectral type of M7.5, it shows Li
absorption and signatures of accretion, which implies that it still has a disk
and suggests an age below 10 Myr. The space motion vector and position on the
sky indicate that the brown dwarf is probably a member of the 20 Myr old
Tuc-Hor association, or that it may be an ejected member of the 12 Myr
old Pic association, both would imply that 2MASS J0041353562112 may
in fact be older than 10 Myr. No accreting star or brown dwarf was previously
known in these associations. Assuming an age of 10 Myr, the brown dwarf has a
mass of about 30 M and is located at 35 pc distance. The newly
discovered object is the closest accreting brown dwarf known. Its membership to
an association older than 10 Myr implies that either disks in brown dwarfs can
survive as long as in more massive stars, perhaps even longer, or that star
formation in Tuc-Hor or Pic occured more recently than previously
thought. The history and evolution of this object can provide new fundamental
insight into the formation process of stars, brown dwarfs, and planets.Comment: emulateapj version, online version at
http://www.iop.org/EJ/abstract/1538-4357/702/2/L119
Characterizing the IYJ Excess Continuum Emission in T Tauri Stars
We present the first characterization of the excess continuum emission of
accreting T Tauri stars between optical and near-infrared wavelengths. With
nearly simultaneous spectra from 0.48 to 2.4 microns acquired with HIRES and
NIRSPEC on Keck and SpeX on the IRTF, we find significant excess continuum
emission throughout this region, including the I, Y, and J bands, which are
usually thought to diagnose primarily photospheric emission. The IYJ excess
correlates with the excess in the V band, attributed to accretion shocks in the
photosphere, and the excess in the K band, attributed to dust in the inner disk
near the dust sublimation radius, but it is too large to be an extension of the
excess from these sources. The spectrum of the excess emission is broad and
featureless, suggestive of blackbody radiation with a temperature between 2200
and 5000 K. The luminosity of the IYJ excess is comparable to the accretion
luminosity inferred from modeling the blue and ultraviolet excess emission and
may require reassessment of disk accretion rates. The source of the IYJ excess
is unclear. In stars of low accretion rate, the size of the emitting region is
consistent with cooler material surrounding small hot accretion spots in the
photosphere. However, for stars with high accretion rates, the projected area
is comparable to or exceeds that of the stellar surface. We suggest that at
least some of the IYJ excess emission arises in the dust-free gas inside the
dust sublimation radius in the disk.Comment: Accepted to ApJ, 31 pages, 21 figure
The Evolution of Circumstellar Disks Surrounding Intermediate Mass Stars: IC 1805
We report the results of a study of the intermediate and high mass stars in
the young, rich star-forming complex IC 1805, based on a combination of
optical, near-infrared, and mid-infrared photometry, and classification
spectra. These data provide the basis for characterizing the masses and ages
for stars more massive than ~2 Msun and enable a study of the frequency and
character of circumstellar disks associated with intermediate- and high-mass
stars. Optically thick accretion disks among stars with masses 2 < M/Msun <4
are rare (~2% of members) and absent among more massive stars. A larger
fraction (~10%) of stars with masses 2 < M/Msun < 4 appear to be surrounded by
disks that have evolved from the initial optically thick accretion phase. We
identify four classes of such disks. These classes are based on spectral energy
distributions (SEDs) of excess emsission above photospheric levels: disks that
are (1) optically thin based on the magnitude of the observed excess emission
from 2 to 24 um; (2) optically thin in their inner regions (r< 20 AU) and
optically thick in their outer regions; (3) exhibit empty inner regions (r < 10
AU) and optically thin emission in their outer regions; and (4) exhibit empty
inner regions and optically thick outer regions. We discuss, and assess the
merits and liabilities of, proposed explanations for disks exhibiting these SED
types and suggest additional observations that would test these proposals.Comment: Accepted to Ap
Demographics of Transition Objects
The unusual properties of transition objects (young stars with an optically
thin inner disc surrounded by an optically thick outer disc) suggest that
significant disc evolution has occured in these systems. We explore the nature
of these systems by examining their demographics, specifically their stellar
accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T
Tauri stars of comparable age. We find that transition objects in Taurus occupy
a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition
single stars in Taurus, they have stellar accretion rates that are typically
~10 times lower at the same disc mass and median disc masses ~4 times larger.
These properties are anticipated by several proposed planet formation theories
and suggest that the formation of Jovian mass planets may play a significant
role in explaining the origin of at least some transition objects. Considering
transition objects as a distinct demographic group among accreting T Tauri
stars leads to a tighter relationship between disc masses and stellar accretion
rates, with a slope between the two quantities that is close to the value of
unity expected in simple theories of disc accretion.Comment: 11 pages, 4 figures, to appear in MNRA
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