472 research outputs found
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
Connection between jets, winds and accretion in T Tauri stars: the X-shooter view
We have analysed the [OI]6300 A line in a sample of 131 young stars with
discs in the Lupus, Chamaeleon and signa Orionis star forming regions, observed
with the X-shooter spectrograph at VLT. The stars have mass accretion rates
spanning from 10^{-12} to 10^{-7} Mo/yr. The line profile was deconvolved into
a low velocity component (LVC,
40 km/s ), originating from slow winds and high velocity jets, respectively.
The LVC is by far the most frequent component, with a detection rate of 77%,
while only 30% of sources have a HVC. The [OI]6300 luminosity of both the LVC
and HVC, when detected, correlates with stellar and accretion parameters of the
central sources (i.e. Lstar , Mstar , Lacc , Macc), with similar slopes for the
two components. The line luminosity correlates better with the accretion
luminosity than with the stellar luminosity or stellar mass. We suggest that
accretion is the main drivers for the line excitation and that MHD disc-winds
are at the origin of both components. In the sub-sample of Lupus sources
observed with ALMA a relationship is found between the HVC peak velocity and
the outer disc inclination angle, as expected if the HVC traces jets ejected
perpendicularly to the disc plane. Mass loss rates measured from the HVC span
from ~ 10^{-13} to ~10^{-7} Mo/yr. The corresponding Mloss/Macc ratio ranges
from ~0.01 to ~0.5, with an average value of 0.07. However, considering the
upper limits on the HVC, we infer a ratio < 0.03 in more than 40% of sources.
We argue that most of these sources might lack the physical conditions needed
for an efficient magneto-centrifugal acceleration in the star-disc interaction
region. Systematic observations of populations of younger stars, that is, class
0/I, are needed to explore how the frequency and role of jets evolve during the
pre-main sequence phase.Comment: 15 pages, 14 figures, Accepted for publication in A&
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants
Extensive surveys of star-forming regions with Spitzer have revealed
populations of disk-bearing young stellar objects. These have provided crucial
constraints, such as the timescale of dispersal of protoplanetary disks,
obtained by carefully combining infrared data with spectroscopic or X-ray data.
While observations in various regions agree with the general trend of
decreasing disk fraction with age, the Lupus V and VI regions appeared to have
been at odds, having an extremely low disk fraction. Here we show, using the
recent Gaia data release 2 (DR2), that these extremely low disk fractions are
actually due to a very high contamination by background giants. Out of the 83
candidate young stellar objects (YSOs) in these clouds observed by Gaia, only
five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and
have similar proper motions to other members in this star-forming complex. Of
these five targets, four have optically thick (Class II) disks. On the one
hand, this result resolves the conundrum of the puzzling low disk fraction in
these clouds, while, on the other hand, it further clarifies the need to
confirm the Spitzer selected diskless population with other tracers, especially
in regions at low galactic latitude like Lupus V and VI. The use of Gaia
astrometry is now an independent and reliable way to further assess the
membership of candidate YSOs in these, and potentially other, star-forming
regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects
We present the results of a study of the stellar and accretion properties of
the (almost) complete sample of class II and transitional YSOs in the Lupus I,
II, III and IV clouds, based on spectroscopic data acquired with the
VLT/X-Shooter spectrograph. Our study combines the dataset from our previous
work with new observations of 55 additional objects. We have investigated 92
YSO candidates in total, 11 of which have been definitely identified with giant
stars unrelated to Lupus. The stellar and accretion properties of the 81 bona
fide YSOs, which represent more than 90% of the whole class~II and transition
disc YSO population in the aforementioned Lupus clouds, have been homogeneously
and self-consistently derived, allowing for an unbiased study of accretion and
its relationship with stellar parameters. The accretion luminosity, Lacc,
increases with the stellar luminosity, Lstar, with an overall slope of ~1.6,
similar but with a smaller scatter than in previous studies. There is a
significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always
lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to
observational biases, but is a true property of the Lupus YSOs. The
logMacc-logMstar correlation shows a statistically significant evidence of a
break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher
masses. The bimodality of the Macc-Mstar relation is confirmed with four
different evolutionary models used to derive the stellar mass. The bimodal
behaviour of the observed relationship supports the importance of modelling
self-gravity in the early evolution of the more massive discs, but other
processes, such as photo evaporation and planet formation during the YSO's
lifetime, may also lead to disc dispersal on different timescales depending on
the stellar mass. We also refined the empirical Lacc vs. Lline relationships.Comment: 43 pages, 22 figure
A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II
We have obtained low- and medium-resolution optical spectra of 20 candidate
young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud,
using the Magellan Baade telescope. We analyze these data in conjunction with
near-infrared photometry from the 2-Micron All Sky Survey. We find that one
target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of
forbidden emission lines. These signatures are usually associated with
accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also
reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it.
Therefore, we propose that C41 is a classical T Tauri object near the
sub-stellar boundary. Thirteen other targets in our sample have continuum
spectra without intrinsic absorption or emission features, and are difficult to
characterize. They may be background giants or foreground field stars not
associated with the cloud or embedded protostars, and need further
investigation. The six remaining candidates, with moderate reddening, are
likely to be older field dwarfs, given their spectral types, lack of lithium
and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200
Evolving temporal association rules with genetic algorithms
A novel framework for mining temporal association rules by discovering itemsets with a genetic algorithm is introduced. Metaheuristics have been applied to association rule mining, we show the efficacy of extending this to another variant - temporal association rule mining. Our framework is an enhancement to existing temporal association rule mining methods as it employs a genetic algorithm to simultaneously search the rule space and temporal space. A methodology for validating the ability of the proposed framework isolates target temporal itemsets in synthetic datasets. The Iterative Rule Learning method successfully discovers these targets in datasets with varying levels of difficulty
X-Shooter study of accretion in Chamaeleon I: II. A steeper increase of accretion with stellar mass for very low mass stars?
The dependence of the mass accretion rate on the stellar properties is a key
constraint for star formation and disk evolution studies. Here we present a
study of a sample of stars in the Chamaeleon I star forming region carried out
using the VLT/X-Shooter spectrograph. The sample is nearly complete down to
M~0.1Msun for the young stars still harboring a disk in this region. We derive
the stellar and accretion parameters using a self-consistent method to fit the
broad-band flux-calibrated medium resolution spectrum. The correlation between
the accretion luminosity to the stellar luminosity, and of the mass accretion
rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3,
respectively. These slopes and the accretion rates are consistent with previous
results in various star forming regions and with different theoretical
frameworks. However, we find that a broken power-law fit, with a steeper slope
for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller
than ~ 0.3 Msun, is slightly preferred according to different statistical
tests, but the single power-law model is not excluded. The steeper relation for
lower mass stars can be interpreted as a faster evolution in the past for
accretion in disks around these objects, or as different accretion regimes in
different stellar mass ranges. Finally, we find two regions on the mass
accretion versus stellar mass plane empty of objects. One at high mass
accretion rates and low stellar masses, which is related to the steeper
dependence of the two parameters we derived. The second one is just above the
observational limits imposed by chromospheric emission. This empty region is
located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be
effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with
the one expected for photoevaporation to rapidly dissipate the inner disk.Comment: Accepted for publication on Astronomy & Astrophysics. Abstract
shortened for arxiv constraints. Revised version after language editin
2MASS Observations of the Perseus, Orion A, Orion B, and Monoceros R2 Molecular Clouds
We use the 2MASS Second Incremental Release Point Source Catalog to
investigate the spatial distribution of young stars in the Perseus, Orion A,
Orion B, and MonR2 molecular clouds. After subtracting a semi-empirical model
of the field star contamination from the observed star counts, stellar surface
density maps are used to identify compact clusters and any stellar population
found more uniformly distributed over the molecular cloud. Each cloud contains
between 2 to 7 clusters, with at least half of the cluster population found in
a single, rich cluster. In addition, a distributed stellar population is
inferred in the Orion A and MonR2 molecular clouds within the uncertainties of
the field star subtraction with a surface density between 0.013 - 0.083
arcmin**-2. The fraction of the total stellar population contained in clusters
for the nominal extinction model ranges from ~50-100% if the distributed
population is relatively young (< 10 Myr), to ~25%-70% if it is relatively old
(~100 Myr). The relatively high fraction of stars contained in clusters
regardless of the age of the distributed population, in conjunction with the
young ages generally inferred for embedded clusters in nearby molecular clouds,
indicates that a substantial fraction of the total stellar population in these
regions has formed within the past few million years in dense clusters. This
suggests that either the star formation rate in each these clouds has recently
peaked if one assumes clouds have ages > 10 Myr, or molecular clouds are
younger than typically thought if one assumes that the star formation rate has
been approximately constant in time.Comment: to appear in AJ, December 2000; see also
http://astro.caltech.edu/~jmc/papers/2mass_cloud
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