365 research outputs found
Temperature constraints on the coldest brown dwarf known WISE 0855-0714
Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE08550714, located 2.2 pc away, and to
improve its temperature estimate (T= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the band. For BDs with T500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3- level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE08550714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T250\,K,.Comment: 4 pages, 4 figures. A&A letter Accepte
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region
Context. Most observational studies so far point towards brown dwarfs sharing
a similar formation mechanism as the one accepted for low mass stars. However,
larger databases and more systematic studies are needed before strong
conclusions can be reached. Aims. In this second paper of a series devoted to
the study of the spectroscopic properties of the members of the Lambda Orionis
Star Forming Region, we study accretion, activity and rotation for a wide set
of spectroscopically confirmed members of the central star cluster Collinder 69
to draw analogies and/or differences between the brown dwarf and stellar
populations of this cluster. Moreover, we present comparisons with other star
forming regions of similar and different ages to address environmental effects
on our conclusions. Methods. We study prominent photospheric lines to derive
rotational velocities and emission lines to distinguish between accretion
processes and chromospheric activity. In addition, we include information about
disk presence and X-ray emission. Results. We report very large differences in
the disk fractions of low mass stars and brown dwarfs (~58%) when compared to
higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for
this dichotomy. As a byproduct, we address the implications of the spatial
distribution of disk and diskless members in the formation scenario of the
cluster itself. We have used the Halpha emission to discriminate among
accreting and non-accreting sources finding that 38+8-7% of sources harboring
disks undergo active accretion and that his percentage stays similar in the
substellar regime. For those sources we have estimated accretion rates.
Finally, regarding rotational velocities, we find a high dispersion in vsin(i)
which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and
an online tabl
Large amplitude periodic outbursts and long period variables in the VVV VIRAC2- database
© 2022 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stac768The VISTA Variables in the Via Lactea (VVV) survey obtained near-infrared photometry toward the Galactic bulge and the southern disc plane for a decade (2010 - 2019). We designed a modified Lomb-Scargle method to search for large-amplitude (Ks > 1.5 mag) mid to long-term periodic variables (P > 10 d) in the 2nd version of VVV Infrared Astrometric Catalogue (VIRAC2-). In total, 1520 periodic sources were discovered, including 59 candidate periodic outbursting young stellar objects (YSOs), based on the unique morphology of the phase-folded light curves, proximity to Galactic HII regions and mid-infrared colours. Five sources are spectroscopically confirmed as accreting YSOs. Both fast-rise/slow-decay and slow-rise/fast-decay periodic outbursts were found, but fast-rise/slow-decay outbursts predominate at the highest amplitudes. The multi-wavelength colour variations are consistent with a variable mass accretion process, as opposed to variable extinction. The cycles are likely to be caused by dynamical perturbations from stellar or planetary companions within the circumstellar disc. An additional search for periodic variability amongst YSO candidates in published Spitzer-based catalogues yielded a further 71 candidate periodic accretors, mostly with lower amplitudes. These resemble cases of pulsed accretion but with unusually long periods and greater regularity. The majority of other long-period variables are pulsating dusty Miras with smooth and symmetric light curves. We find that some Miras have redder colours than previously thought, most likely due to their surface chemical compositions.Peer reviewedFinal Accepted Versio
Search for associations containing young stars (SACY) VIII. An updated census of spectroscopic binary systems showing hints of non-universal multiplicity among these associations
We seek to update the spectroscopy binary fraction of the SACY (Search for
Associations Containing Young stars) sample taking in consideration all
possible biases in our identification of binary candidates, such as activity
and rotation. Using high-resolution spectroscopic observations we have produced
1300 cross-correlation functions (CCFs) to disentangle the previously
mentioned sources of contamination. The radial velocity values obtained were
cross-matched with the literature and were used to revise and update the
spectroscopic binary (SB) fraction in each of the SACY association. In order to
better describe the CCF profile, we calculated a set of high-order
cross-correlation features to determine the origin of the variations in radial
velocities. We identified 68 SB candidates from our sample of 410 objects. Our
results hint that the youngest associations have a higher SB fraction.
Specifically, we found sensitivity-corrected SB fractions of for ~Cha , for TW Hya and
for ~Pictoris, in contrast with the five
oldest ( Myr) associations we have sampled which are
or lower. This result seems independent of the methodology used to asses
membership to the associations. The new CCF analysis, radial velocity estimates
and SB candidates are particularly relevant for membership revision of targets
in young stellar associations. These targets would be ideal candidates for
follow-up campaigns using high-resolution techniques in order to confirm
binarity, resolve the orbits, and ideally calculate dynamical masses.
Additionally, if the results on SB fraction in the youngest associations are
confirmed, it could hint of non-universal multiplicity among SACY associations.Comment: The paper has been accepted in A&
Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope
observing program obtained the first extensive mid-infrared (3.6 & 4.5 um)
time-series photometry of the Orion Nebula Cluster plus smaller footprints in
eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,
L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).
There are ~29,000 unique objects with light curves in either or both IRAC
channels in the YSOVAR data set. We present the data collection and reduction
for the Spitzer and ancillary data, and define the "standard sample" on which
we calculate statistics, consisting of fast cadence data, with epochs about
twice per day for ~40d. We also define a "standard sample of members",
consisting of all the IR-selected members and X-ray selected members. We
characterize the standard sample in terms of other properties, such as spectral
energy distribution shape. We use three mechanisms to identify variables in the
fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and
significant periodicity. We also identified variables on the longest timescales
possible of ~6 years, by comparing measurements taken early in the Spitzer
mission with the mean from our YSOVAR campaign. The fraction of members in each
cluster that are variable on these longest timescales is a function of the
ratio of Class I/total members in each cluster, such that clusters with a
higher fraction of Class I objects also have a higher fraction of long-term
variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,
we find that a star with a longer period is more likely than those with shorter
periods to have an IR excess. We do not find any evidence for variability that
causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members
and field objects combined, at most 0.02% may have transient IR excesses.Comment: Accepted to AJ; 38 figures, 93 page
Multiwavelength detection of an ongoing FUOr-type outburst on a low-mass YSO
©2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/During the pre-main-sequence evolution, Young Stellar Objects (YSOs) assemble most of their mass during the episodic accretion process. The rarely seen FUOr-type events are valuable laboratories to investigate the outbursting nature of YSOs. Here, we present multiwavelength detection of a high-amplitude eruptive source in the young open cluster VdBH 221 with an ongoing outburst, including optical to mid-infrared time series and near-infrared spectra. The initial outburst has an exceptional amplitude of >6.3 mag in Gaia and 4.6 mag in Ks, with a peak luminosity up to 16 Lâ and a peak mass accretion rate of 1.4 Ă 10-5 Mâ yr-1. The optical to infrared spectral energy distribution of this object is consistent with a low-mass star (0.2 Mâ) with a modest extinction (AV < 2 mag). A 100-d delay between optical and infrared rising stages is detected, suggesting an outside-in origin of the instability. The spectroscopic features of this object reveal a self-luminous accretion disc, very similar to FU Orionis, with a low line-of-sight extinction. Most recently, there has been a gradual increase in brightness throughout the wavelength range, possibly suggesting an enhancement of the mass accretion rate.Peer reviewe
Characterizing the morphology of the debris disk around the low-mass star GSC 07396-00759
Context. Debris disks have commonly been studied around intermediate-mass
stars. Their intense radiation fields are believed to efficiently remove the
small dust grains that are constantly replenished by collisions. For lower-mass
stars, in particular M-stars, the dust removal mechanism needs to be further
investigated given the much weaker radiation field produced by these objects.
Aims. We present new polarimetric observations of the nearly edge-on disk
around the pre-main sequence M-type star GSC 07396-00759, taken with VLT/SPHERE
IRDIS, with the aim to better understand the morphology of the disk, its dust
properties, and the star-disk interaction via the stellar mass-loss rate.
Methods. We model our observations to characterize the location and properties
of the dust grains using the Henyey-Greenstein approximation of the polarized
phase function and evaluate the strength of the stellar winds. Results. We find
that the observations are best described by an extended and highly inclined
disk (i\approx 84.3\,^{\circ}\pm0.3) with a dust distribution centered at a
radius au. The polarized phase function is best
reproduced by an anisotropic scattering factor and small
micron-sized dust grains with sizes m. We furthermore
discuss some of the caveats of the approach and a degeneracy between the grain
size and the porosity. Conclusions. Even though the radius of the disk may be
over-estimated, our results suggest that using a given scattering theory might
not be sufficient to fully explain key aspects such as the shape of the phase
function, or the dust grain size. With the caveats in mind, we find that the
average mass-loss rate of GSC 07396-00759 can be up to 500 times stronger than
that of the Sun, supporting the idea that stellar winds from low-mass stars can
evacuate small dust grains from the disk
Herschel -PACS observations of [OI] and H2O in Cha II
Gas plays a major role in the dynamical evolution of protoplanetary discs.
Its coupling with the dust is the key to our understanding planetary formation.
Studying the gas content is therefore a crucial step towards understanding
protoplanetary discs evolution. Such a study can be made through spectroscopic
observations of emission lines in the far-infrared, where some of the most
important gas coolants emit, such as the [OI] 3P1-3 P2 transition at 63.18
microns. We aim at characterising the gas content of protoplanetary discs in
the intermediate-aged Chamaeleon II (Cha II) star forming region. We also aim
at characterising the gaseous detection fractions within this age range, which
is an essential step tracing gas evolution with age in different star forming
regions. We obtained Herschel-PACS line scan spectroscopic observations at 63
microns of 19 Cha II Class I and II stars. The observations were used to trace
[OI] and o-H2O at 63 microns. The analysis of the spatial distribution of [OI],
when extended, can be used to understand the origin of the emission. We have
detected [OI] emission toward seven out of the nineteen systems observed, and
o-H2O emission at 63.32 microns in just one of them, Sz 61. Cha II members show
a correlation between [OI] line fluxes and the continuum at 70 microns, similar
to what is observed in Taurus. We analyse the extended [OI] emission towards
the star DK Cha and study its dynamical footprints in the PACS Integral Field
Unit (IFU). We conclude that there is a high velocity component from a jet
combined with a low velocity component with an origin that may be a combination
of disc, envelope and wind emission. The stacking of spectra of objects not
detected individually in [OI] leads to a marginal 2.6sigma detection that may
indicate the presence of gas just below our detection limits for some, if not
all, of them.Comment: 10 pages, 7 figure
The Gaia -ESO Survey : Empirical determination of the precision of stellar radial velocities and projected rotation velocities
Context. The Gaia-ESO Survey (GES) is a large public spectroscopic survey at the European Southern Observatory Very Large Telescope. Aims. A key aim is to provide precise radial velocities (RVs) and projected equatorial velocities (v sin i) for representative samples of Galactic stars, which will complement information obtained by the Gaia astrometry satellite. Methods. We present an analysis to empirically quantify the size and distribution of uncertainties in RV and v sin i using spectra from repeated exposures of the same stars. Results. We show that the uncertainties vary as simple scaling functions of signal-to-noise ratio (S/N) and v sin i, that the uncertainties become larger with increasing photospheric temperature, but that the dependence on stellar gravity, metallicity and age is weak. The underlying uncertainty distributions have extended tails that are better represented by Student's t-distributions than by normal distributions. Conclusions. Parametrised results are provided, which enable estimates of the RV precision for almost all GES measurements, and estimates of the v sin i precision for stars in young clusters, as a function of S/N, v sin i and stellar temperature. The precision of individual high S/N GES RV measurements is 0.22-0.26 kms-1, dependent on instrumental configuration.Peer reviewedFinal Accepted Versio
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