197 research outputs found
Young stars in Epsilon Cha and their disks: disk evolution in sparse associations
(abridge) The nearby young stellar association Epsilon Cha association has an
estimated age of 3-5 Myr, making it an ideal laboratory to study the disk
dissipation process and provide empirical constraints on the timescale of
planet formation. We combine the available literature data with our Spitzer IRS
spectroscopy and VLT/VISIR imaging data. The very low mass stars USNO-B120144.7
and 2MASS J12005517 show globally depleted spectral energy distributions
pointing at strong dust settling. 2MASS J12014343 may have a disk with a very
specific inclination where the central star is effectively screened by the cold
outer parts of a flared disk but the 10 micron radiation of the warm inner disk
can still reach us. We find the disks in sparse stellar associations are
dissipated more slowly than those in denser (cluster) environments. We detect
C_{2}H_{2} rovibrational band around 13.7 micron on the IRS spectrum of
USNO-B120144.7. We find strong signatures of grain growth and crystallization
in all Epsilon Cha members with 10 micron features detected in their IRS
spectra. We combine the dust properties derived in the Epsilon Cha sample with
those found using identical or similar methods in the MBM 12, Coronet cluster,
Eta Cha associations, and in the cores to disks (c2d) legacy program. We find
that disks around low-mass young stars show a negative radial gradient in the
mass-averaged grain size and mass fraction of crystalline silicates. A positive
correlation exists between the mass-averaged grain sizes of amorphous silicates
and the accretion rates if the latter is above ~10^{-9} Msun/yr, possibly
indicating that those disks are sufficiently turbulent to prevent grains of
several microns in size to sink into the disk interior.Comment: 17 pages, 18 figures, 6 tables, language revised; accepted to A&
Disk Evolution in OB Associations - Deep Spitzer/IRAC Observations of IC 1795
We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried
out to investigate the evolution of protoplanetary disks in regions of massive
star formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we
find 289 cluster members. An additional 340 sources with an infrared excess,
but without X-ray counterpart, are classified as cluster member candidates.
Both surveys are complete down to stellar masses of about 1 Msun. We present
pre-main sequence isochrones computed for the first time in the Spitzer/IRAC
colors. The age of the cluster, determined via the location of the Class III
sources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3
- 5 Myr. As theoretically expected, we do not find any systematic variation in
the spatial distribution of disks within 0.6 pc of either O-type star in the
association. However, the disk fraction in IC 1795 does depend on the stellar
mass: sources with masses >2 Msun have a disk fraction of ~20%, while lower
mass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks
around massive stars have a shorter dissipation timescale.Comment: Accepted for publication in Ap
Time-resolved photometry of the young dipper RX~J1604.3-2130A:Unveiling the structure and mass transport through the innermost disk
Context. RX J1604.3-2130A is a young, dipper-type, variable star in the Upper Scorpius association, suspected to have an inclined inner disk, with respect to its face-on outer disk. Aims. We aim to study the eclipses to constrain the inner disk properties. Methods. We used time-resolved photometry from the Rapid Eye Mount telescope and Kepler 2 data to study the multi-wavelength variability, and archival optical and infrared data to track accretion, rotation, and changes in disk structure. Results. The observations reveal details of the structure and matter transport through the inner disk. The eclipses show 5 d quasi-periodicity, with the phase drifting in time and some periods showing increased/decreased eclipse depth and frequency. Dips are consistent with extinction by slightly processed dust grains in an inclined, irregularly-shaped inner disk locked to the star through two relatively stable accretion structures. The grains are located near the dust sublimation radius (similar to 0.06 au) at the corotation radius, and can explain the shadows observed in the outer disk. The total mass (gas and dust) required to produce the eclipses and shadows is a few % of a Ceres mass. Such an amount of mass is accreted/replenished by accretion in days to weeks, which explains the variability from period to period. Spitzer and WISE infrared variability reveal variations in the dust content in the innermost disk on a timescale of a few years, which is consistent with small imbalances (compared to the stellar accretion rate) in the matter transport from the outer to the inner disk. A decrease in the accretion rate is observed at the times of less eclipsing variability and low mid-IR fluxes, confirming this picture. The v sin i = 16 km s(-1) confirms that the star cannot be aligned with the outer disk, but is likely close to equator-on and to be aligned with the inner disk. This anomalous orientation is a challenge for standard theories of protoplanetary disk formation.Science & Technology Facilities Council (STFC): ST/S000399/1.
ESO fellowship.
European Union (EU): 823 823.
German Research Foundation (DFG): FOR 2634/1 TE 1024/1-1.
French National Research Agency (ANR): ANR-16-CE31-0013.
Alexander von Humboldt Foundation.
European Research Council (ERC): 678 194.
European Research Council (ERC): 742 095.
National Aeronautics & Space Administration (NASA).
National Science Foundation (NSF).
National Aeronautics & Space Administration (NASA): NNG05GF22G.
National Science Foundation (NSF): AST-0909182, AST-1 313 422
MBM 12: young protoplanetary discs at high galactic latitude
(abridged) We present Spitzer infrared observations to constrain disc and
dust evolution in young T Tauri stars in MBM 12, a star-forming cloud at high
latitude with an age of 2 Myr and a distance of 275 pc. The region contains 12
T Tauri systems, with primary spectral types between K3 and M6; 5 are weak-line
and the rest classical T Tauri stars. We first use MIPS and literature
photometry to compile spectral energy distributions for each of the 12 members
in MBM 12, and derive their IR excesses. The IRS spectra are analysed with the
newly developed two-layer temperature distribution (TLTD) spectral
decomposition method. For the 7 T Tauri stars with a detected IR excess, we
analyse their solid-state features to derive dust properties such as
mass-averaged grain size, composition and crystallinity. We find a spatial
gradient in the forsterite to enstatite range, with more enstatite present in
the warmer regions. The fact that we see a radial dependence of the dust
properties indicates that radial mixing is not very efficient in the discs of
these young T Tauri stars. The SED analysis shows that the discs in MBM 12, in
general, undergo rapid inner disc clearing, while the binary sources have
faster discevolution. The dust grains seem to evolve independently from the
stellar properties, but are mildly related to disc properties such as flaring
and accretion rates.Comment: 14 pages, accepted by Astronomy and Astrophysic
Metallicities of Young Open Clusters I: NGC 7160 and NGC 2232
We present a moderate-resolution spectroscopic analysis of the 10-25 Myr
clusters NGC 7160 and NGC 2232, using observations obtained with the WIYN 3.5-m
telescope. Both NGC 7160 and NGC 2232 are found to have super-solar
metallicities, with a mean [Fe/H] = 0.16 \pm 0.03 (s.e.m.) for NGC 7160, and
0.22 \pm 0.09 (s.e.m.) or 0.32 \pm 0.08 for NGC 2232, depending on the adopted
temperature scale. NGC 7160 exhibits solar distributions of Na, Fe-peak, and
{\alpha}-elements. NGC 2232 is underabundant in light elements Al and Si, by
~0.25 and ~ 0.15 dex, respectively; [Ni/Fe] is roughly solar. The abundance of
lithium in NGC 2232 stars is in agreement with undepleted values reported for
other cluster main sequence stars. Our abundances are similar to other
metal-rich open clusters and Galactic thin and thick disk stars.Comment: Accepted for publication in The Astronomical Journal. 10 figures, 11
tables. Full versions of the data tables can be made available upon email
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The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region
Rich young stellar clusters produce HII regions whose expansion into the
nearby molecular cloud is thought to trigger the formation of new stars.
However, the importance of this mode of star formation is uncertain. This
investigation seeks to quantify triggered star formation (TSF) in IC 1396A
(a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the
periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37
cluster. X-ray selection of young stars from Chandra X-ray Observatory data is
combined with existing optical and infrared surveys to give a more complete
census of the TSF population. Over 250 young stars in and around IC 1396A are
identified; this doubles the previously known population. A spatio-temporal
gradient of stars from the IC 1396A cloud toward the primary ionizing star HD
206267 is found. We argue that the TSF mechanism in IC 1396A is the
radiation-driven implosion process persisting over several million years.
Analysis of the X-ray luminosity and initial mass functions indicates that >140
stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC
1396 HII region, we estimate the TSF contribution for the entire HII region
exceeds 14-25% today, and may be higher over the lifetime of the HII region.
Such triggering on the periphery of HII regions may be a significant mode of
star formation in the Galaxy.Comment: Accepted for publication in MNRAS; 28 pages, 18 figure
The bipolar outflow and disk of the brown dwarf ISO217
We show that the very young brown dwarf candidate ISO217 (M6.25) is driving
an intrinsically asymmetric bipolar outflow with a stronger and slightly faster
red-shifted component based on spectro-astrometry of forbidden [SII] emission
lines observed in UVES/VLT spectra taken in 2009. ISO217 is only one of a
handful of brown dwarfs and VLMS (M5-M8) for which an outflow has been detected
and that show that the T Tauri phase continues at the substellar limit. We
measure a spatial extension of the outflow of +/-190mas (+/-30AU) and
velocities of +/-40-50kms/s. We show that the velocity asymmetry between both
lobes is variable on timescales of a few years and that the strong asymmetry of
a factor of 2 found in 2007 might be smaller than originally anticipated when
using a more realistic stellar rest-velocity. We also detect forbidden
[FeII]7155 emission, for which we propose as potential origin the hot inner
regions of the outflow. To understand the ISO217 system, we determine the disk
properties based on radiative transfer modeling of the SED. This disk model
agrees very well with Herschel/PACS data at 70mu. We find that the disk is
flared and intermediately inclined (~45deg). The total disk mass (4e-6 Msun) is
small compared to the accretion and outflow rate of ISO217 (~1e-10 Msun/yr). We
propose that this discrepancy can be explained by either a higher disk mass
than inferred from the model (strong undetected grain growth) and/or by an on
average lower accretion and outflow rate than the determined values. We show
that a disk inclination significantly exceeding 45deg, as suggested from Halpha
modeling and from both lobes of the outflow being visible, is inconsistent with
the SED data. Thus, despite its intermediate inclination angle, the disk of
this brown dwarf does not appear to obscure the red outflow component, which is
very rarely seen for T Tauri objects (only one other case).Comment: Accepted for publication at A&A; minor changes (language editing
U-band study of the accretion properties in the sigma Ori star-forming region
This paper presents the results of an U band survey with FORS1/VLT of a large
area in the sigma Orionis star-forming region. We combine the U-band photometry
with literature data to compute accretion luminosity and mass accretion rates
from the U-band excess emission for all objects (187) detected by Spitzer in
the FORS1 field and classified by Hernandez et al. (2007) as likely members of
the cluster. The sample stars range in mass from ~0.06 to ~1.2 Msun; 72 of them
show evidence of disks and we measure mass accretion rates Macc between
<10^{-11} and few 10^{-9} Msun/yr, using the colors of the diskless stars as
photospheric templates. Our results confirm the dependence of Macc on the mass
of the central object, which is stronger for low-mass stars and flattens out
for masses larger than ~0.3 Msun; the spread of Macc for any value of the
stellar mass is ~2 orders of magnitude. We discuss the implications of these
results in the context of disk evolution models. Finally, we analyze the
relation between Macc and the excess emission in the Spitzer bands, and find
that at Macc ~10^{-10} Msun/yr the inner disks change from optically thin to
optically thick.Comment: 19 pages, 17 figures, accepted by A&
High-Resolution Spectroscopy in Tr37: Gas Accretion Evolution in Evolved Dusty Disks
Using the Hectochelle multifiber spectrograph, we have obtained
high-resolution (R~34,000) spectra in the Halpha region for a large number of
stars in the 4 Myr-old cluster Tr 37, containing 146 previously known members
and 26 newly identified ones. We present the Halpha line profiles of all
members, compare them to our IR observations of dusty disks (2MASS/JHK + IRAC +
MIPS 24 micron), use the radial velocities as a membership criterion, and
calculate the rotational velocities. We find a good correlation between the
accretion-broadened profiles and the presence of protoplanetary disks, noting
that a small fraction of the accreting stars presents broad profiles with
Halpha equivalent widths smaller than the canonical limit separating CTTS and
WTTS. The number of strong accretors appears to be lower than in younger
regions, and a large number of CTTS have very small accretion rates
(dM/dt<10^{-9} Msun/yr). Taking into account that the spectral energy
distributions are consistent with dust evolution (grain growth/settling) in the
innermost disk, this suggests a parallel evolution of the dusty and gaseous
components. We also observe that about half of the "transition objects" (stars
with no IR excesses at wavelengths shorter than ~6 micron) do not show any
signs of active accretion, whereas the other half is accreting with accretion
rates <10^{-9} Msun/yr. These zero or very low accretion rates reveal important
gas evolution and/or gas depletion in the innermost disk, which could be
related to grain growth up to planetesimal or even planet sizes. Finally, we
examine the rotational velocities of accreting and non accreting stars, finding
no significant differences that could indicate disk locking at these ages.Comment: 51 pages, 13 (reduced resolution) figures, 2 tables. AJ in pres
The Long-Lived Disks in the Eta Chamaeleontis Cluster
We present IRS spectra and revised MIPS photometry for the 18 members of the
Eta Chamaeleontis cluster. Aged 8 Myr, the Eta Cha cluster is one of the few
nearby regions within the 5-10 Myr age range, during which the disk fraction
decreases dramatically and giant planet formation must come to an end. For the
15 low-mass members, we measure a disk fraction ~50%, high for their 8 Myr age,
and 4 of the 8 disks lack near-IR excesses, consistent with the empirical
definition of "transition'' disks. Most of the disks are comparable to
geometrically flat disks. The comparison with regions of different ages
suggests that at least some of the "transition" disks may represent the normal
type of disk around low-mass stars. Therefore, their flattened structure and
inner holes may be related to other factors (initial masses of the disk and the
star, environment, binarity), rather than to pure time evolution. We analyze
the silicate dust in the disk atmosphere, finding moderate crystalline
fractions (~10-30%) and typical grain sizes ~1-3 micron, without any
characteristic trend in the composition. These results are common to other
regions of different ages, suggesting that the initial grain processing occurs
very early in the disk lifetime (<1 Myr). Large grain sizes in the disk
atmosphere cannot be used as a proxy for age, but are likely related to higher
disk turbulence. The dust mineralogy varies between the 8-12micron and the
20-30 micron features, suggesting high temperature dust processing and little
radial mixing. Finally, the analysis of IR and optical data on the B9 star Eta
Cha reveals that it is probably surrounded by a young debris disk with a large
inner hole, instead of being a classical Be star.Comment: 35 pages, 6 tables, 8 figures; Accepted by Ap
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