439 research outputs found
Accretion properties of T Tauri stars in sigma Ori
Accretion disks around young stars evolve in time with time scales of few
million years. We present here a study of the accretion properties of a sample
of 35 stars in the ~3 million year old star-forming region sigma Ori. Of these,
31 are objects with evidence of disks, based on their IR excess emission. We
use near-IR hydrogen recombination lines (Pa_gamma) to measure their mass
accretion rate. We find that the accretion rates are significantly lower in
sigma Ori than in younger regions, such as rho-Oph, consistently with viscous
disk evolution. The He I 1.083 micron line is detected (either in absorption or
in emission) in 72% of the stars with disks, providing evidence of
accretion-powered activity also in very low accretors, where other accretion
indicators dissapear.Comment: Astronomy and Astrophysics, accepte
Choquet integral-based aggregation of interface usability parameters: identification of fuzzy measure
Disk evolution in the Ori OB1 association
We analyze multi-band photometry of a subsample of low mass stars in the
associations Ori OB1a and 1b discovered during the CIDA Orion Variability
Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained
UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26
Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b.
We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and
18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected
at 10um while only one was detected at 18um. We estimate mass accretion rates
from the excess luminosity at U, and find that they are consistent with
determinations for a number of other associations, with or without high mass
star formation. The observed decrease of mass accretion rate with age is
qualitatively consistent with predictions of viscous evolution of accretion
disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to
Ori OB1a. This decrease implies that significant grain growth and settling
towards the midplane has taken place in the inner disks of Ori OB1. We compare
the SED of the star detected at both 10um and 18um with disk models for similar
stellar and accretion parameters. We find that the low <= 18 um fluxes of this
Ori OB1b star cannot be due to the smaller disk radius expected from viscous
evolution in the presence of the FUV radiation fields from the OB stars in the
association. Instead, we find that the disk of this star is essentially a flat
disk, with little if any flaring, indicating a a significant degree of dust
settling towards the midplane, as expected from dust evolution in
protoplanetary disks.Comment: 35 pages, 11 figures, to appear in the Astronomical Journal. Full
resolution figures in http://www.cida.ve/~briceno/publications
Slow Cooling of an Ising Ferromagnet
A ferromagnetic Ising chain which is endowed with a single-spin-flip Glauber
dynamics is investigated. For an arbitrary annealing protocol, we derive an
exact integral equation for the domain wall density. This integral equation
admits an asymptotic solution in the limit of extremely slow cooling. For
instance, we extract an asymptotic of the density of domain walls at the end of
the cooling procedure when the temperature vanishes. Slow annealing is usually
studied using a Kibble-Zurek argument; in our setting, this argument leads to
approximate predictions which are in good agreement with exact asymptotics.Comment: 6 page
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 Possible Detection of Occultation by a Proto-planetary Clump in GM Cephei
GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an
abrupt variable and to have a circumstellar disk with very active accretion.
Our monitoring observations in 2009-2011 revealed the star to show sporadic
flare events, each with brightening of < 0.5 mag lasting for days. These
brightening events, associated with a color change toward the blue, should
originate from an increased accretion activity. Moreover, the star also
underwent a brightness drop of ~1 mag lasting for about a month, during which
the star became bluer when fainter. Such brightness drops seem to have a
recurrence time scale of a year, as evidenced in our data and the photometric
behavior of GM Cep over a century. Between consecutive drops, the star
brightened gradually by about 1 mag and became blue at peak luminosity. We
propose that the drop is caused by obscuration of the central star by an
orbiting dust concentration. The UX Orionis type of activity in GM Cep
therefore exemplifies the disk inhomogeneity process in transition between
grain coagulation and planetesimal formation in a young circumstellar disk.Comment: In submission to the Astrophysical Journal, 4 figure
Impact of grain evolution on the chemical structure of protoplanetary disks
We study the impact of dust evolution in a protoplanetary disk around a T
Tauri star on the disk chemical composition. For the first time we utilize a
comprehensive model of dust evolution which includes growth, fragmentation and
sedimentation. Specific attention is paid to the influence of grain evolution
on the penetration of the UV field in the disk. A chemical model that includes
a comprehensive set of gas phase and grain surface chemical reactions is used
to simulate the chemical structure of the disk. The main effect of the grain
evolution on the disk chemical composition comes from sedimentation, and, to a
lesser degree, from the reduction of the total grain surface area. The net
effect of grain growth is suppressed by the fragmentation process which
maintains a population of small grains, dominating the total grain surface
area. We consider three models of dust properties. In model GS both growth and
sedimentation are taken into account. In models A5 and A4 all grains are
assumed to have the same size (10(-5) cm and 10(-4) cm, respectively) with
constant gas-to-dust mass ratio of 100. Like in previous studies, the
"three-layer" pattern (midplane, molecular layer, hot atmosphere) in the disk
chemical structure is preserved in all models, but shifted closer to the
midplane in models with increased grain size (GS and A4). Unlike other similar
studies, we find that in models GS and A4 column densities of most gas-phase
species are enhanced by 1-3 orders of magnitude relative to those in a model
with pristine dust (A5), while column densities of their surface counterparts
are decreased. We show that column densities of certain species, like C2H,
HC(2n+1)N (n=0-3), H2O and some other molecules, as well as the C2H2/HCN
abundance ratio which are accessible with Herschel and ALMA can be used as
observational tracers of early stages of the grain evolution process in
protoplanetary disks.Comment: 50 pages, 4 tables, 11 figures, accepted to the Ap
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
Protostars, multiplicity, and disk evolution in the Corona Australis region: a <i>Herschel</i> Gould Belt Study
Context. The CrA region and the Coronet cluster form a nearby (138 pc), young (1-2 Myr) star-forming region that hosts a moderate population of Class I, II, and III objects.
Aims: We study the structure of the cluster and the properties of the protostars and protoplanetary disks in the region.
Methods: We present Herschel PACS photometry at 100 and 160 ÎŒm, obtained as part of the Herschel Gould Belt Survey. The Herschel maps reveal the cluster members within the cloud with high sensitivity and high dynamic range.
Results: Many of the cluster members are detected, including some embedded, very low-mass objects, several protostars (some of them extended), and substantial emission from the surrounding molecular cloud. Herschel also reveals some striking structures, such as bright filaments around the IRS 5 protostar complex and a bubble-shaped rim associated with the Class I object IRS 2. The disks around the Class II objects display a wide range of mid- and far-IR excesses consistent with different disk structures. We have modeled the disks with the RADMC radiative transfer code to quantify their properties. Some of them are consistent with flared, massive, relatively primordial disks (S CrA, T CrA). Others display significant evidence for inside-out evolution, consistent with the presence of inner holes/gaps (G-85, G-87). Finally, we found disks with a dramatic small dust depletion (G-1, HBC 677) that, in some cases, could be related to truncation or to the presence of large gaps in a flared disk (CrA-159). The derived masses for the disks around the low-mass stars are found to be below the typical values in Taurus, in agreement with previous Spitzer observations.
Conclusions: The Coronet cluster presents itself as an interesting compact region that contains both young protostars and very evolved disks. The Herschel data provide sufficient spatial resolution to detect small-scale details, such as filamentary structures or spiral arms associated with multiple star formation. The disks around the cluster members range from massive, flared primordial disks to disks with substantial small dust grain depletion or with evidence of inside-out evolution. This results in an interesting mixture of objects for a young and presumably coevally formed cluster. Given the high degree of multiplicity and interactions observed among the protostars in the region, the diversity of disks may be a consequence of the early star formation history, which should also be taken into account when studying the disk properties in similar sparsely populated clusters
Candidate Coronagraphic Detections of Protoplanetary Disks around Four Young Stars
We present potential detections of H-band scattered light emission around
four young star, selected from a total sample of 45 young stars observed with
the CIAO coronagraph of the Subaru telescope. Two CTTS, CI Tau and DI Cep, and
two WTTS, LkCa 14 and RXJ 0338.3+1020 were detected. In all four cases, the
extended emission is within the area of the residual PSF halo, and is revealed
only through careful data reduction. We compare the observed extended emission
with simulations of the scattered light emission, to evaluate the plausibility
and nature of the detected emission.Comment: 9 Figures, 40 page
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