450 research outputs found
Gaia view of low-mass star formation
Understanding how young stars and their circumstellar disks form and evolve
is key to explain how planets form. The evolution of the star and the disk is
regulated by different processes, both internal to the system or related to
their environment. The former include accretion of material onto the central
star, wind emission, and photoevaporation of the disk due to high-energy
radiation from the central star. These are best studied spectroscopically, and
the distance to the star is a key parameter in all these studies. Here we
present new estimates of the distance to a complex of nearby star-forming
clouds obtained combining TGAS distances with measurement of extinction on the
line of sight. Furthermore, we show how we plan to study the effects of the
environment on the evolution of disks with Gaia, using a kinematic modelling
code we have developed to model young star-forming regions.Comment: 4 pages, 1 figure. To appear in the Proceedings of IAU Symposium 330:
Astrometry and Astrophysics in the Gaia Sk
Identification of new transitional disk candidates in Lupus with Herschel
New data from the Herschel Space Observatory are broadening our understanding
of the physics and evolution of the outer regions of protoplanetary disks in
star forming regions. In particular they prove to be useful to identify
transitional disk candidates. The goals of this work are to complement the
detections of disks and the identification of transitional disk candidates in
the Lupus clouds with data from the Herschel Gould Belt Survey. We extracted
photometry at 70, 100, 160, 250, 350 and 500 m of all spectroscopically
confirmed Class II members previously identified in the Lupus regions and
analyzed their updated spectral energy distributions. We have detected 34 young
disks in Lupus in at least one Herschel band, from an initial sample of 123
known members in the observed fields. Using the criteria defined in Ribas et
al. (2013) we have identified five transitional disk candidates in the region.
Three of them are new to the literature. Their PACS-70 m fluxes are
systematically higher than those of normal T Tauri stars in the same
associations, as already found in T Cha and in the transitional disks in the
Chamaeleon molecular cloud. Herschel efficiently complements mid-infrared
surveys for identifying transitional disk candidates and confirms that these
objects seem to have substantially different outer disks than the T Tauri stars
in the same molecular clouds.Comment: Accepted for publication in A&A. 16 pages, 9 figures, 7 table
The Gaia mission
Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page
A reconsideration of disk properties in Herbig Ae stars
This paper presents state-of-the-art spectral energy distributions (SEDs) of four Herbig Ae stars, based in part on new data in the mid and far-infrared and at millimeter wavelengths. The SEDs are discussed in the context of circumstellar disk models. We show that models of irradiated disks provide a good fit to the observations over the whole range of wavelengths. We offer a possible solution to the long-standing puzzle caused by the excess emission of Herbig Ae stars, where a large fraction of the stellar luminosity is re-radiated between ~1.25 and 7 μm, with a peak at about 3 μm. We suggest that this general behaviour can be caused by dust evaporation in disks where the gas component is optically thin to the stellar radiation, as expected if the accretion rate is very low. The creation of a puffed-up inner wall of optically thick dust at the dust sublimation radius can account for the near-infrared characteristics of the SEDs. It can also naturally explain the H and K band interferometric observations of AB Aur (Millan-Gabet et al. [CITE]), which reveal a ring of emission of radius ~0.3 AU. Finally, irradiated disk models can easily explain the observed intensity of the 10 μm silicate features and their variation from star to star
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: Organisation and challenges for the data processing
Gaia is an ambitious space astrometry mission of ESA with a main objective to
map the sky in astrometry and photometry down to a magnitude 20 by the end of
the next decade. While the mission is built and operated by ESA and an
industrial consortium, the data processing is entrusted to a consortium formed
by the scientific community, which was formed in 2006 and formally selected by
ESA one year later. The satellite will downlink around 100 TB of raw telemetry
data over a mission duration of 5 years from which a very complex iterative
processing will lead to the final science output: astrometry with a final
accuracy of a few tens of microarcseconds, epoch photometry in wide and narrow
bands, radial velocity and spectra for the stars brighter than 17 mag. We
discuss the general principles and main difficulties of this very large data
processing and present the organisation of the European Consortium responsible
for its design and implementation.Comment: 7 pages, 2 figures, Proceedings of IAU Symp. 24
On the degree of stochastic asymmetry in the tidal tails of star clusters
Context: Tidal tails of star clusters are commonly understood to be populated
symmetrically. Recently, the analysis of Gaia data revealed large asymmetries
between the leading and trailing tidal tail arms of the four open star clusters
Hyades, Praesepe, Coma Berenices and NGC 752. Aims: As the evaporation of stars
from star clusters into the tidal tails is a stochastic process, the degree of
stochastic asymmetry is quantified in this work. Methods: For each star cluster
1000 configurations of test particles are integrated in the combined potential
of a Plummer sphere and the Galactic tidal field over the life time of the
particular star cluster. For each of the four star clusters the distribution
function of the stochastic asymmetry is determined and compared with the
observed asymmetry. Results: The probabilities for a stochastic origin of the
observed asymmetry of the four star clusters are: Praesepe ~1.7 sigma, Coma
Berenices ~2.4 sigma, Hyades ~6.7 sigma, NGC 752 ~1.6 sigma. Conclusions: In
the case of Praesepe, Coma Berenices and NGC 752 the observed asymmetry can be
interpreted as a stochastic evaporation event. However, for the formation of
the asymmetric tidal tails of the Hyades additional dynamical processes beyond
a pure statistical evaporation effect are required.Comment: accepted for publication by A&
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