599 research outputs found
REM near-IR and optical photometric monitoring of Pre-Main Sequence Stars in Orion
We performed an intensive photometric monitoring of the PMS stars falling in
a field of about 10x10 arc-minutes in the vicinity of the Orion Nebula Cluster
(ONC). Photometric data were collected between November 2006 and January 2007
with the REM telescope in the VRIJHK' bands. The largest number of observations
is in the I band (about 2700 images) and in J and H bands (about 500 images in
each filter). From the observed rotational modulation, induced by the presence
of surface inhomogeneities, we derived the rotation periods for 16 stars and
improved previous determinations for the other 13. The analysis of the spectral
energy distributions and, for some stars, of high-resolution spectra provided
us with the main stellar parameters (luminosity, effective temperature, mass,
age, and vsini). We also report the serendipitous detection of two strong
flares in two of these objects. In most cases, the light-curve amplitudes
decrease progressively from the R to H band as expected for cool starspots,
while in a few cases, they can only be modelled by the presence of hot spots,
presumably ascribable to magnetospheric accretion. The application of our own
spot model to the simultaneous light curves in different bands allowed us to
deduce the spot parameters and particularly to disentangle the spot temperature
and size effects on the observed light curves.Comment: 29 pages, 24 figure
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&
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
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 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
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
A Three Micron Survey of the Chamaeleon I Dark Cloud
We describe an L-band photometric survey of 0.5 square deg of the Cha I dark
cloud. The survey has a completeness limit of L < 11.0. Our survey detects 124
sources, including all known pre-main sequence stars with L < 11. The fraction
of sources with near-IR excess emission is 58% +- 4% for K = 9-11. Cha I
sources have bluer H-K and K-L colors than pre-main sequence stars in
Taurus-Auriga. These sources also have a strong correlation between EW(H-alpha)
and K-L. Stars with K-L 0.6
have strong H-alpha emission. Because many Cha I sources are heavily reddened,
this division between weak emission T Tauri stars and classical T Tauri stars
occurs at a redder K-L than in Taurus-Auriga.Comment: 12 pages of text, 4 figures, and 1 three page table of data modified
version adds reference and acknowledgemen
The origin of R CrA variability: A complex triple system hosting a disk
R~CrA is the brightest member of the Coronet star forming region and it is
the closest Herbig AeBe star with a spectrum dominated by emission lines. Its
luminosity has been monitored since the end of the 19th century, but the origin
of its variability, which shows a stable period of ~days, is
still unknown. We studied photometric and spectroscopic data for this star to
investigate the nature of the variability of R~CrA. We exploited the fact that
near infrared luminosity of the Herbig AeBe stars is roughly proportional to
the total luminosity of the stars to derive the absorption, and then mass and
age of R~CrA. In addition, we model the periodic modulation of the light curve
as due to partial attenuation of a central binary by a circumbinary disk. This
model reproduces very well the observations. We found that the central object
in R~CrA is a very young (~Myr), highly absorbed (~mag) binary; we obtain masses of ~M and
~M for the two components. We propose that the
secular decrease of the R~CrA apparent luminosity is due to a progressive
increase of the disk absorption. This might be related to precession of a
slightly inclined disk caused by the recently discovered M-dwarf companion.
Thus, R~CrA may be a triple system hosting a disk.Comment: Accepted for publication in A&A. 14 pages, 11 figure
The VLT/NaCo Large program to probe the occurrence of exoplanets and brown dwarfs in wide orbits: I- Sample definition and characterization
Young, nearby stars are ideal targets to search for planets using the direct
imaging technique. The determination of stellar parameters is crucial for the
interpretation of imaging survey results particularly since the luminosity of
substellar objects has a strong dependence on system age. We have conducted a
large program with NaCo at the VLT in order to search for planets and brown
dwarfs in wide orbits around 86 stars. A large fraction of the targets observed
with NaCo were poorly investigated in the literature. We performed a study to
characterize the fundamental properties (age, distance, mass) of the stars in
our sample. To improve target age determinations, we compiled and analyzed a
complete set of age diagnostics. We measured spectroscopic parameters and age
diagnostics using dedicated observations acquired with FEROS and CORALIE
spectrographs at La Silla Observatory. We also made extensive use of archival
spectroscopic data and results available in the literature. Additionally, we
exploited photometric time-series, available in ASAS and Super-WASP archives,
to derive rotation period for a large fraction of our program stars. We
provided updated characterization of all the targets observed in the VLT NaCo
Large program, a survey designed to probe the occurrence of exoplanets and
brown dwarfs in wide orbits. The median distance and age of our program stars
are 64 pc and 100 Myr, respectively. Nearly all the stars have masses between
0.70 and 1.50sun, with a median value of 1.01 Msun. The typical metallicity is
close to solar, with a dispersion that is smaller than that of samples usually
observed in radial velocity surveys. Several stars are confirmed or proposed
here to be members of nearby young moving groups. Eight spectroscopic binaries
are identified.Comment: 64 pages with Appendix, 15 figures, accepted to A&
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