170 research outputs found
The double population of Chamaeleon I detected by Gaia DR2
Context. Chamaeleon I represents an ideal laboratory to study the cluster
formation in a low-mass environment. Recently, two sub clusters spatially
located in the northern and southern parts of Chamaeleon I were found with
different ages and radial velocities. Aims. In this letter we report new
insights into the structural properties, age, and distance of Chamaeleon I
based on the astrometric parameters from Gaia data-release 2 (DR2). Methods. We
identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We
determined the median distance of the cluster using Gaia parallaxes and fitted
the distribution of parallaxes and proper motions assuming the presence of two
clusters. We derived the probability of each single source of belonging to the
northern or southern sub-clusters, and compared the HR diagram of the most
probable members to pre-main sequences isochrones. Results. The median distance
of Chamaeleon I is ~190 pc. This is about 20 pc larger than the value commonly
adopted in the literature. From a Kolmogorov-Smirnov test of the parallaxes and
proper-motion distributions we conclude that the northern and southern clusters
do not belong to the same parent population. The northern population has a
distance dN = 192.7+/-0.4 pc, while the southern one dS = 186.5+/-0.7 pc. The
two sub-clusters appear coeval, at variance with literature results, and most
of the sources are younger than 3 Myr. The northern cluster is more elongated
and extends towards the southern direction partially overlapping with the more
compact cluster located in the south. A hint of a relative rotation between the
two sub-clusters is also found.Comment: Letter accepted by A&
Reanalysis of the FEROS observations of HIP 11952
Aims. We reanalyze FEROS observations of the star HIP 11952 to reassess the
existence of the proposed planetary system. Methods. The radial velocity of the
spectra were measured by cross-correlating the observed spectrum with a
synthetic template. We also analyzed a large dataset of FEROS and HARPS
archival data of the calibrator HD 10700 spanning over more than five years. We
compared the barycentric velocities computed by the FEROS and HARPS pipelines.
Results. The barycentric correction of the FEROS-DRS pipeline was found to be
inaccurate and to introduce an artificial one-year period with a semi-amplitude
of 62 m/s. Thus the reanalysis of the FEROS data does not support the existence
of planets around HIP 11952.Comment: 7 pages, 8 figures, 1 tabl
A network of filaments detected by Herschel in the Serpens core : a laboratory to test simulations of low-mass star formation
V.R. was partly supported by the DLR grant number 50 OR 1109 and by the Bayerische Gleichstellungsförderung (BGF). This research was partly supported by the Priority Programme 1573 âPhysics of the Interstellar Mediumâ of the German Science Foundation (DFG), the DFG cluster of excellence âOrigin and Structure of the Universeâ and by the Italian Ministero dellâIstruzione, UniversitĂ e Ricerca through the grant Progetti Premiali 2012 -iALMA (CUP C52I13000140001). C.E. is partly supported by Spanish Grants AYA 2011-26202 and AYA 2014-55840-P.Context. Filaments represent a key structure during the early stages of the star formation process. Simulations show that filamentary structures commonly formed before and during the formation of cores. Aims. The Serpens core is an ideal laboratory for testing the state of the art of simulations of turbulent giant molecular clouds. Methods. We used Herschel observations of the Serpens core to compute temperatureand column density maps of the region. We selected the early stages of are cent simulation of star-formation, before stellar feedback was initiated, with similar total mass and physical size as the Serpens core. We also derived temperature and column density maps from the simulations. The observed distribution of column densities of the filaments was analyzed, first including and then masking the cores. The same analysis was performed on the simulations as well. Results. A radial network of filaments was detected in the Serpens core. The analyzed simulation shows a striking morphological resemblance to the observed structures. The column density distribution of simulated filaments without cores shows only a log-normal distribution, while the observed filaments show a power-law tail. The power-law tail becomes evident in the simulation if the focus is only the column density distribution of the cores. In contrast, the observed cores show a flat distribution. Conclusions. Even though the simulated and observed filaments are subjectively similar-looking, we find that they behave in very different ways. The simulated filaments are turbulence-dominated regions; the observed filaments are instead self-gravitating structures that will probably fragment into cores.Publisher PDFPeer reviewe
A 3D view of the Taurus star-forming region by <i>Gaia </i>and <i>Herschel</i>:multiple populations related to the filamentary molecular cloud
Context. Taurus represents an ideal region to study the three-dimensional distribution of the young stellar population and relate it to the associated molecular cloud. Aims. The second Gaia data release (DR2) enables us to investigate the Taurus complex in three dimensions, starting from a previously defined robust membership. The molecular cloud structured in filaments can be traced in emission using the public far-infrared maps from Herschel. Methods. From a compiled catalog of spectroscopically confirmed members, we analyze the 283 sources with reliable parallax and proper motions in the Gaia DR2 archive. We fit the distribution of parallaxes and proper motions with multiple populations described by multivariate Gaussians. We compute the cartesian Galactic coordinates (X,Y,Z) and, for the populations associated with the main cloud, also the galactic space velocity (U,V,W). We discuss the spatial distribution of the populations in relation to the structure of the filamentary molecular cloud traced by Herschel. Results. We discover the presence of six populations which are all well defined in parallax and proper motions, with the only exception being Taurus D. The derived distances range between 130 and 160 pc. We do not find a unique relation between stellar population and the associated molecular cloud: while the stellar population seems to be on the cloud surface, both lying at similar distances, this is not the case when the molecular cloud is structured in filaments. Taurus B is probably moving in the direction of Taurus A, while Taurus E appears to be moving towards them. Conclusions. The Taurus region is the result of a complex star formation history which most probably occurred in clumpy and filamentary structures that are evolving independently
Aldebaran's angular diameter: how well do we know it?
The bright, well-known K5 giant Aldebaran, alpha Tau, is probably the star
with the largest number of direct angular diameter determinations, achieved
over a long time by several authors using various techniques. In spite of this
wealth of data, or perhaps as a direct result of it, there is not a very good
agreement on a single angular diameter value. This is particularly unsettling
if one considers that Aldebaran is also used as a primary calibrator for some
angular resolution methods, notably for optical and infrared long baseline
interferometry. Directly connected to Aldebaran's angular diameter and its
uncertainties is its effective temperature, which also has been used for
several empirical calibrations. Among the proposed explanations for the
elusiveness of an accurate determination of the angular diameter of Aldebaran
are the possibility of temporal variations as well as a possible dependence of
the angular diameter on the wavelength. We present here a few, very accurate
new determinations obtained by means of lunar occultations and long baseline
interferometry. We derive an average value of 19.96+-0.03 milliarcseconds for
the uniform disk diameter. The corresponding limb-darkened value is 20.58+-0.03
milliarcseconds, or 44.2+-0.9 R(sun). We discuss this result, in connection
with previous determinations and with possible problems that may affect such
measurements.Comment: 8 pages, 4 figures, accepted for publication in A&
The Herschel/PACS view of the Cep OB2 region: Global protoplanetary disk evolution and clumpy star formation
Astronomy and Astrophysics 573 (2015): A19 reproduced with permission from Astronomy & AstrophysicsContext. The Cep OB2 region, with its two intermediate-aged clusters Tr 37 and NGC7160, is a paradigm of sequential star formation and an ideal site for studies of protoplanetary disk evolution. Aims. We use Herschel data to study the protoplanetary disks and the star formation history of the region. Methods. Herschel/PACS observations at 70 and 160 ÎŒm probe the disk properties (mass, dust sizes, structure) and the evolutionary state of a large number of young stars. Far-IR data also trace the remnant cloud material and small-scale cloud structure. Results. We detect 95 protoplanetary disks at 70 ÎŒm, 41 at 160 ÎŒm, and obtain upper limits for more than 130 objects. The detection fraction at 70 ÎŒm depends on the spectral type (88% for K4 or earlier stars, 17% for M3 or later stars) and on the disk type (âŒ50% for full and pre-transitional disks, âŒ35% for transitional disks, no low-excess/depleted disks detected). Non-accreting disks are not detected, suggesting significantly lower masses. Accreting transition and pre-transition disks have systematically higher 70 ÎŒm excesses than full disks, suggestive of more massive, flared and/or thicker disks. Herschel data also reveal several mini-clusters in Tr 37, which are small, compact structures containing a few young stars surrounded by nebulosity. Conclusions. Far-IR data are an excellent probe of the evolution of disks that are too faint for sub-millimetre observations. We find a strong link between far-IR emission and accretion, and between the inner and outer disk structure. Herschel confirms the dichotomy between accreting and non-accreting transition disks. Accretion is a powerful measure of global disk evolution: substantial mass depletion and global evolution need to occur to shut down accretion in a protoplanetary disk, even if the disk has inner holes. Disks likely follow different evolutionary paths: low disk masses do not imply opening inner holes, and having inner holes does not require low disk masses. The mini-clusters reveal multi-episodic star formation in Tr 37. The long survival of mini-clusters suggest that they formed from the fragmentation of the same core. Their various morphologies favour different formation/triggering mechanisms acting within the same cluster. The beads-on-a-string structure in one mini-cluster is consistent with gravitational fragmentation or gravitational focusing, acting on very small scales (solar-mass stars in âŒ0.5 pc filaments). Multi-episodic star formation could also produce evolutionary variations between disks in the same region. Finally, Herschel also unveils what could be the first heavy mass loss episode of the O6.5 star HD206267 in Tr 37A.S.A. acknowledges support by the Spanish MICINN/MINECO âRamĂłn y Cajalâ program, grant number RYC-2010-06164. A.S.A. and M.F. acknowledge support by the
action âProyectos de InvestigaciĂłn fundamental no orientadaâ, grant number AYA2012-35008. C.E. is partly supported by Spanish MICINN/MINECO grant AYA2011-26202. V.R. is supported by the DLR grant number 50 OR 1109 and by the Bayerischen Gleichstellungsförderung (BGF). T.B. acknowledges support from NASA Origins of Solar Systems grant NNX12AJ04G. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, Franc
Tests of stellar model atmospheres by optical interferometry III: NPOI and VINCI interferometry of the M0 giant gamma Sge covering 0.5 - 2.2 microns
Aims: We present a comparison of the visual and NIR intensity profile of the
M0 giant gamma Sagittae to plane-parallel ATLAS 9 as well as to plane-parallel
& spherical PHOENIX model atmospheres. Methods: We use previously described
visual interferometric data obtained with the NPOI in July 2000. We apply the
recently developed technique of coherent integration, and thereby obtain
visibility data of more spectral channels and with higher precision than
before. In addition, we employ new measurements of the K-band diameter of gamma
Sagittae obtained with the instrument VINCI at the VLTI in 2002. Results: The
spherical PHOENIX model leads to a precise definition of the Rosseland angular
diameter and a consistent high-precision diameter value for our NPOI and
VLTI/VINCI data sets of Theta_Ross=6.06 pm 0.02 mas, with the Hipparcos
parallax corresponding to R_Ross=55 pm 4 R_sun, and with the bolometric flux
corresponding to an effective temperature T_eff=3805 pm 55 K. Our visual
visibility data close to the first minimum and in the second lobe constrain the
limb-darkening effect and are generally consistent with the model atmosphere
predictions. The visual closure phases exhibit a smooth transition between 0
and pi. Conclusions: The agreement between the NPOI and VINCI diameter values
increases the confidence in the model atmosphere predictions from optical to
NIR wavelengths as well as in the calibration and accuracy of both
interferometric facilities. The consistent night-by-night diameter values of
VINCI give additional confidence in the given uncertainties. The closure phases
suggest a slight deviation from circular symmetry, which may be due to surface
features, an asymmetric extended layer, or a faint unknown companion.Comment: 12 pages, 9 figures, accepted by A&A. Also available from
http://www.aanda.org/articles/aa/pdf/forth/aa5853_06.pd
Mid-infrared observations of the transitional disks around DH Tau, DM Tau, and GM Aur
Aims: We present mid-infrared observations and photometry of the transitional
disks around the young stellar objects DH Tau, DM Tau, and GM Aur, obtained
with VISIR/VLT in N band. Our aim is to resolve the inner region and the
large-scale structures of these transitional disks, carrying potential
signatures of intermediate or later stages of disk evolution and ongoing planet
formation.
Methods: We use the simultaneously observed standard-stars as PSF reference
to constrain the radial flux profiles of our target objects. Subtracting the
obtained standard-star profile from the corresponding science object profile
yields the flux residuals produced by the star-disk system. A detection
threshold takes into account the background standard deviation and also the
seeing variations during the observations to evaluate the significance of these
flux residuals. On the basis of a simple model for the dust re-emission, we
derive constraints on the inner radius of the dust disk.
Results: We spatially resolve the transitional disk around GM Aur and
determine an inner-disk hole radius of 20.5(+1.0,-0.5) AU. The circumstellar
disks around DH Tau and DM Tau are not spatially resolved but we are able to
constrain the inner-disk hole radius to <15.5(+9.0,-2.0) AU and
<15.5(+0.5,-0.5) AU, respectively. The performed photometry yields fluxes of
178+-31 mJy for DH Tau, 56+-6 mJy for DM Tau, and 229+-14 mJy for GM Aur.Comment: Accepted for publication in Astronomy & Astrophysics. (6 pages,
including 7 figures and 5 tables
Planetary companions around the metal-poor star HIP 11952
Aims. We carried out a radial-velocity survey to search for planets around
metal-poor stars. In this paper we report the discovery of two planets around
HIP 11952, a metal-poor star with [Fe/H]= -1.9 that belongs to our target
sample. Methods. Radial velocity variations of HIP 11952 were monitored
systematically with FEROS at the 2.2 m telescope located at the ESO La Silla
observatory from August 2009 until January 2011. We used a cross-correlation
technique to measure the stellar radial velocities (RV). Results. We detected a
long-period RV variation of 290 d and a short-period one of 6.95 d. The
spectroscopic analysis of the stellar activity reveals a stellar rotation
period of 4.8 d. The Hipparcos photometry data shows intra-day variabilities,
which give evidence for stellar pulsations. Based on our analysis, the observed
RV variations are most likely caused by the presence of unseen planetary
companions. Assuming a primary mass of 0.83 M\odot, we computed minimum
planetary masses of 0.78 MJup for the inner and 2.93 MJup for the outer planet.
The semi-major axes are a1 = 0.07 AU and a2 = 0.81 AU, respectively.
Conclusions. HIP 11952 is one of very few stars with [Fe/H]< -1.0 which have
planetary companions. This discovery is important to understand planet
formation around metal-poor starsComment: Published in A&
Squeezed between shells? On the origin of the Lupus I molecular cloud. - II. APEX CO and GASS HI observations
Accepted for publication in a future issue of Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.Context. The Lupus I cloud is found between the Upper-Scorpius (USco) and the Upper-Centaurus-Lupus (UCL) sub-groups of the Scorpius-Centaurus OB-association, where the expanding USco H I shell appears to interact with a bubble currently driven by the winds of the remaining B-stars of UCL. Aims. We investigate if the Lupus I molecular could have formed in a colliding flow, and in particular, how the kinematics of the cloud might have been influenced by the larger scale gas dynamics. Methods. We performed APEX 13CO(2â1) and C 18O(2â1) line observations of three distinct parts of Lupus I that provide kinematic information on the cloud at high angular and spectral resolution. We compare those results to the atomic hydrogen data from the GASS H i survey and our dust emission results presented in the previous paper. Based on the velocity information, we present a geometric model for the interaction zone between the USco shell and the UCL wind bubble. Results. We present evidence that the molecular gas of Lupus I is tightly linked to the atomic material of the USco shell. The CO emission in Lupus I is found mainly at velocities between vLSR = 3â6 km sâ1 which is in the same range as the H i velocities. Thus, the molecular cloud is co-moving with the expanding USco atomic H i shell. The gas in the cloud shows a complex kinematic structure with several line-of-sight components that overlay each other. The non-thermal velocity dispersion is in the transonic regime in all parts of the cloud and could be injected by external compression. Our observations and the derived geometric model agree with a scenario where Lupus I is located in the interaction zone between the USco shell and the UCL wind bubble. Conclusions. The kinematics observations are consistent with a scenario where the Lupus I cloud formed via shell instabilities. The particular location of Lupus I between USco and UCL suggests that counter-pressure from the UCL wind bubble and pre-existing density enhancements, perhaps left over from the gas stream that formed the stellar subgroups, may have played a role in its formation.Peer reviewedFinal Accepted Versio
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