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 $\theta^{1}$ Ori C at the center of the Orion Nebula Cluster (ONC). The object was known to have extended emission pointing away from $\theta^{1}$ Ori C, indicating ongoing external photoevaporation. Our near-infrared spectroscopic data suggests that the central source of Proplyd 133-353 is substellar ($\sim$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 $\theta^{1}$ 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