154 research outputs found
A revised estimate of the distance to the clouds in the Chamaeleon complex using the Tycho-Gaia Astrometric Solution
The determination of the distance to dark star-forming clouds is a key
parameter to derive the properties of the cloud itself, and of its stellar
content. This parameter is still loosely constrained even in nearby
star-forming regions. We want to determine the distances to the clouds in the
Chamaeleon-Musca complex and to explore the connection between these clouds and
the large scale cloud structures in the galaxy. We use the newly estimated
distances obtained from the parallaxes measured by the Gaia satellite and
included in the Tycho-Gaia Astrometric Solution catalog. When known members of
a region are included in this catalog we use their parallaxes to infer the
distance to the cloud. Otherwise, we analyze the dependence of the color excess
on the distance of the stars and look for a turn-on of this excess, which is a
proxy of the position of the front-edge of the star-forming cloud. We are able
to measure the distance to the three Chamaeleon clouds. The distance to
Chamaeleon I is 179 pc, 20 pc further away than previously assumed. The
Chamaeleon II cloud is located at the distance of 181 pc, which agrees with
previous estimates. We are able to measure for the first time a distance to the
Chamaeleon III cloud of 199 pc. Finally, the distance of the Musca cloud is
smaller than 603 pc. These estimates do not allow us to distinguish between the
possibility that the Chamaeleon clouds are part of a sheet of clouds parallel
to the galactic plane, or perpendicular to it. Gaia Data Release 2 will allow
us to put more stringent constraints on the distances to these clouds by giving
us access to parallax measurements for a larger number of members of these
regions.Comment: Accepted for publication on A&A. Abstract shortened for arxiv
constraint
A candidate planetary-mass object with a photoevaporating disk in Orion
In this work, we report the discovery of a candidate planetary-mass object
with a photoevaporating protoplanetary disk, Proplyd 133-353, which is near the
massive star Ori C at the center of the Orion Nebula Cluster
(ONC). The object was known to have extended emission pointing away from
Ori C, indicating ongoing external photoevaporation. Our
near-infrared spectroscopic data suggests that the central source of Proplyd
133-353 is substellar (M9.5), might have a mass probably less than 13
Jupiter mass and an age younger than 0.5 Myr. Proplyd 133-353 shows a similar
ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC
with a similar stellar luminosity, and has a similar proper motion to the mean
one of confirmed ONC members. We propose that Proplyd 133-353 was formed in a
very low-mass dusty cloud near Ori C as a second-generation of
star formation, which can explain both its young age and the presence of its
disk.Comment: 6 pages, 4 figures. Accepted for publication in ApJ
Stellar masses and disk properties of Lupus young stellar objects traced by velocity-aligned stacked ALMA 13CO and C18O spectra
In recent ALMA surveys, the gas distributions and velocity structures of most
of the protoplanetary disks can still not be imaged at high S/N due to the
short integration time. In this work, we re-analyzed the ALMA 13CO (3-2) and
C18O (3-2) data of 88 young stellar objects in Lupus with the velocity-aligned
stacking method to enhance S/N and to study the kinematics and disk properties
traced by molecular lines. This method aligns spectra at different positions in
a disk based on the projected Keplerian velocities at their positions and then
stacks them. This method enhances the S/N ratios of molecular-line data and
allows us to obtain better detections and to constrain dynamical stellar masses
and disk orientations. We obtain 13CO detections in 41 disks and C18O
detections in 18 disks with 11 new detections in 13CO and 9 new detections in
C18O after applying the method. We estimate the disk orientations and the
dynamical stellar masses from the 13CO data. Our estimated dynamical stellar
masses correlate with the spectroscopic stellar masses, and in a subsample of
16 sources, where the inclination angles are better constrained, the two masses
are in a good agreement within the uncertainties and with a mean difference of
0.15 Msun. With more detections of fainter disks, our results show that high
gas masses derived from the 13CO and C18O lines tend to be associated with high
dust masses estimated from the continuum emission. Nevertheless, the scatter is
large (0.9 dex), implying large uncertainties in deriving the disk gas mass
from the line fluxes. We find that with such large uncertainties it is expected
that there is no correlation between the disk gas mass and the mass accretion
rate with the current data. Deeper observations to detect disks with gas masses
<1E-5 Msun in molecular lines are needed to investigate the correlation between
the disk gas mass and the mass accretion rate.Comment: Submitted to A&
Testing the models of X-ray driven photoevaporation with accreting stars in the Orion Nebula Cluster
Recent works highlight the importance of stellar X-rays on the evolution of
the circumstellar disks of young stellar objects, especially for disk
photoevaporation. A signature of this process may be seen in the so far
tentatively observed dependence of stellar accretion rates on X-ray
luminosities. According to models of X-ray driven photoevaporation, stars with
higher X-ray luminosities should show lower accretion rates, on average, in a
sample with similar masses and ages. To this aim, we have analyzed X-ray
properties of young stars in the Orion Nebula Cluster determined with Chandra
during the COUP observation as well as accretion data obtained from the
photometric catalog of the HST Treasury Program. With these data, we have
performed a statistical analysis of the relation between X-ray activity and
accretion rates using partial linear regression analysis. The initial
anticorrelation found with a sample of 332 young stars is considerably weaker
compared to previous studies. However, excluding flaring activity or limiting
the X-ray luminosity to the soft band (0.5 - 2.0 keV) leads to a stronger
anticorrelation, which is statistically more significant. Furthermore, we have
found a weak positive correlation between the higher component of the plasma
temperature gained in the X-ray spectral fitting and the accretion rates,
indicating that the hardness of the X-ray spectra may influence the accretion
process. There is evidence for a weak anticorrelation, as predicted by
theoretical models, suggesting that X-ray photoevaporation modulates the
accretion rate through the inner disk at late stages of disk evolution, leading
to a phase of photoevaporation-starved accretion.Comment: Accepted for publication in Astronomy & Astrophysic
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