38 research outputs found
The Recurrent Eclipse of an Unusual Pre--Main-Sequence Star in IC 348
The recurrence of a previously documented eclipse of a solar-like
pre--main-sequence star in the young cluster IC 348 has been observed. The
recurrence interval is 4.7 yr and portions of 4 cycles have now been
seen. The duration of each eclipse is at least 3.5 years, or % of a
cycle, verifying that this is not an eclipse by a stellar companion. The light
curve is generally symmetric and approximately flat-bottomed. Brightness at
maximum and minimum have been rather stable over the years but the light curve
is not perfectly repetitive or smooth and small variations exist at all phases.
We confirm that the star is redder when fainter. Models are discussed and it is
proposed that this could be a system similar to KH 15D in NGC 2264.
Specifically, it may be an eccentric binary in which a portion of the orbit of
one member is currently occulted during some binary phases by a circumbinary
disk. The star deserves sustained observational attention for what it may
reveal about the circumstellar environment of low-mass stars of planet-forming
age.Comment: Accepted for publication in ApJ (Letters
The Variability and Rotation of Pre-main Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?
A variability study of the young cluster IC 348 at Van Vleck Observatory has
been extended to a total of seven years. Twelve new periodic stars have been
found in the last two years, bringing the total discovered by this program to
40. In addition, we confirm 16 of the periods reported by others and resolve
some discrepancies. The total number of known rotation periods in the cluster,
from all studies has now reached 70. This is sufficient to demonstrate that the
parent population of K5-M2 stars is rotationally indistinguishable from that in
the Orion Nebula Cluster even though their radii are 20% smaller and they would
be expected to spin about twice as fast if angular momentum were conserved. The
median radius and, therefore, inferred age of the IC 348 stars actually closely
matches that of NGC 2264, but the stars spin significantly more slowly. This
suggests that another factor besides mass and age plays a role in establishing
the rotation properties within a cluster and we suggest that it is environment.
If disk locking were to persist for longer times in less harsh environments,
because the disks themselves persist for longer times, it could explain the
generally slower rotation rates observed for stars in this cluster, whose
earliest type star is of class B5. We have also obtained radial velocities, the
first for PMS stars in IC348, and v sin i measurements for 30 cluster stars to
assist in the study of rotation and as an independent check on stellar radii.
Several unusual variable stars are discussed; in some or all cases their
behavior may be linked to occultations by circumstellar material. A strong
correlation exists between the range of photometric variability and the slope
of the spectral energy distribution in the infrared. Nineteen of the 21 stars
with I ranges exceeding 0.4 mag show infrared evidence for circumstellar disks.Comment: Accepted for publication in A
Water emission in NGC1333-IRAS4: The physical structure of the envelope
We report ISO-LWS far infrared observations of CO, water and oxygen lines
towards the protobinary system IRAS4 in the NGC1333 cloud. We detected several
water, OH, CO rotational lines, and two [OI] and [CII] fine structure lines.
Given the relatively poor spectral and spatial resolution of these
observations, assessing the origin of the observed emission is not
straightforward. In this paper, we focus on the water line emission and explore
the hypothesis that it originates in the envelopes that surround the two
protostars, IRAS4 A and B, thanks to an accurate model. The model reproduces
quite well the observed water line fluxes, predicting a density profile, mass
accretion rate, central mass, and water abundance profile in agreement with
previous works. We hence conclude that the emission from the envelopes is a
viable explanation for the observed water emission, although we cannot totally
rule out the alternative that the observed water emission originates in the
outflow
Surface density of the young cluster IC 348 in the Perseus molecular cloud
The IC 348 young star cluster contains more than 300 confirmed members. It is
embedded in the Perseus molecular cloud, making any clustering analysis subject
to an extinction bias. In this work, we derive the extinction map of the cloud
and revisit the content of IC 348 through a statistical approach that uses the
2MASS data. Our goal was to address the question of the completeness of IC 348
and of young clusters in general. We performed a combined analysis of the star
color and density in this region, in order to establish the surface density map
of the cluster. We reached the conclusion that IC 348 has structures up to 25'
from the cluster center, and we estimate that about 40 members brighter than
Ks=13 mag are still unidentified. Although we cannot use our statistical method
to identify these new members individually, the surface density map gives a
strong indication of their actual location. They are distributed in the outer
regions of the cluster, where very few dedicated observations have been made so
far, which is probably why they escaped previous identification. In addition,
we propose the existence of a new embedded cluster associated to the infrared
source MSX6C G160.2784-18.4216, about 38' south of IC 348.Comment: 5 pages, 5 figures, accepted for publication in A&
A Bubbling Nearby Molecular Cloud: COMPLETE Shells in Perseus
We present a study on the shells (and bubbles) in the Perseus molecular cloud
using the COMPLETE survey large-scale 12CO(1-0) and 13CO(1-0) maps. The twelve
shells reported here are spread throughout most of the Perseus cloud and have
circular or arc-like morphologies with a range in radius of about 0.1 to 3 pc.
Most of them have not been detected before most likely as maps of the region
lacked the coverage and resolution needed to distinguish them. The majority of
the shells are coincident with infrared nebulosity of similar shape and have a
candidate powering source near the center. We suggest they are formed by the
interaction of spherical or very wide-angle winds powered by young stars inside
or near the Perseus molecular cloud -a cloud that is commonly considered to be
mostly forming low-mass stars. Two of the twelve shells are powered by
high-mass stars close to the cloud, while the others appear to be powered by
low or intermediate mass stars in the cloud. We argue that winds with a mass
loss rate of about 10^-8 to 10^-6 M_sun/yr are required to produce the observed
shells. Our estimates indicate that the energy input rate from these stellar
winds is similar to the turbulence dissipation rate. We conclude that in
Perseus the total energy input from both collimated protostellar outflows and
powerful spherical winds from young stars is sufficient to maintain the
turbulence in the molecular cloud. Large scale molecular line and IR continuum
maps of a sample of clouds will help determine the frequency of this phenomenon
in other star forming regions.Comment: 48 pages in total: 16 pages of text and references; 2 pages of
tables; 30 figures (one page per figure). Accepted for publication in the
Astrophysical Journa
Physical structure and CO abundance of low-mass protostellar envelopes
We present 1D radiative transfer modelling of the envelopes of a sample of 18
low-mass protostars and pre-stellar cores with the aim of setting up realistic
physical models, for use in a chemical description of the sources. The density
and temperature profiles of the envelopes are constrained from their radial
profiles obtained from SCUBA maps at 450 and 850 micron and from measurements
of the source fluxes ranging from 60 micron to 1.3 mm. The densities of the
envelopes within ~10000 AU can be described by single power-laws r^{-p} for the
class 0 and I sources with p ranging from 1.3 to 1.9, with typical
uncertainties of +/- 0.2. Four sources have flatter profiles, either due to
asymmetries or to the presence of an outer constant density region. No
significant difference is found between class 0 and I sources. The power-law
fits fail for the pre-stellar cores, supporting recent results that such cores
do not have a central source of heating. The derived physical models are used
as input for Monte Carlo modelling of submillimeter C18O and C17O emission. It
is found that class I objects typically show CO abundances close to those found
in local molecular clouds, but that class 0 sources and pre-stellar cores show
lower abundances by almost an order of magnitude implying that significant
depletion occurs for the early phases of star formation. While the 2-1 and 3-2
isotopic lines can be fitted using a constant fractional CO abundance
throughout the envelope, the 1-0 lines are significantly underestimated,
possibly due to contribution of ambient molecular cloud material to the
observed emission. The difference between the class 0 and I objects may be
related to the properties of the CO ices.Comment: 21 pages, 12 figures, accepted by A&
Chandra Study of Young Stellar Objects in the NGC 1333 Star-forming Cloud
NGC 1333, a highly active star formation region within the Perseus molecular
cloud complex, has been observed with the ACIS-I detector on board the Chandra
X-ray Observatory. In our image with a sensitivity limit of ~ 1e28 erg/s, we
detect 127 X-ray sources, of which, 95 are known cluster members. The X-ray
luminosity function of the discovered YSO population spans a range of
1e28-3.2e31 erg/s in the 0.5-8 keV band. Comparison with K-band source counts
indicates that we detect all of the known cluster members with K < 12, and
about half of members with K > 12. We detect seven X-ray emitting YSOs which
drive optically visible jets as well as one deeply embedded object that has not
been detected in near-IR observations. The presence or absence of an outflows
does not appear to produce any difference in X-ray properties of YSOs. We also
find no systematic differences in X-ray luminosity distributions between two
complete subsamples of CTTs and WTTs. That suggests that there is no difference
in the astrophysical mechanism of WTT and CTT X-ray emission production.
Additional results include: the X-ray emission from two late-B stars which
illuminate the reflection nebula originates from unresolved late-type
companions; two T Tauri stars are discovered as previously unknown components
of visual binaries; and the X-ray counterpart of SVS 16 has the column density
much lower than that expected from near-IR photometry and thus its X-ray
luminosity is not anomalously high, as has been previously suggested.Comment: 53 pages, 14 figures, 3 tables. To appear in the Astrophysical
Journal; scheduled for 2002, ApJ, 575 (August 10th). High quality copy
available at ftp://ftp.astro.psu.edu/pub/gkosta/ngc1333
Star formation in Perseus: II. SEDs, classification and lifetimes
Working with the submillimetre continuum map of the Perseus molecular cloud
(Hatchell et al. 2005), we aimed to determine the evolutionary stage of each
submm core in Perseus, and investigate the lifetimes of these phases. We
compile spectral energy distributions (SEDs) from 2MASS, Spitzer IRAC,
Michelle, IRAS, SCUBA and Bolocam data. Sources are classified
starless/protostellar on the basis of infrared and/or outflow detections and
Class I/Class 0 on the basis of Tbol, Lbol/Lsmm and F_{3.6}/F_{850}. In order
to investigate the dependence of these evolutionary indicators on mass, we
construct radiative transfer models of Class 0 sources. Of the submm cores,
56/103 (54%) are confirmed protostars on the basis of infrared emission or
molecular outflows. Of these, 22 are classified Class 1 on the basis of three
evolutionary indicators, 34 are Class 0, and the remaining 47 are assumed
starless. Perseus contains a much greater fraction of Class 0 sources than
either Taurus or Rho Oph. Comparing the protostellar with the T Tauri
population, the lifetime of the protostellar phase in Perseus is 0.25-0.67 Myr
(95% confidence limits). The relative lifetime of the Class 0 and Class 1
phases are similar. We find that for the same source geometry but different
masses, evolutionary indicators such as Tbol vary their value. It is therefore
not always appropriate to use a fixed threshold to separate Class 0 and Class I
sources. More modelling is required to determine the observational
characteristics of the Class 0/Class I boundary over a range of masses.Comment: A&A accepted. 35 pages, 24 figures. Version with original figures
available at http://www.astro.ex.ac.uk/people/hatchell/publications.htm
Young T-Dwarf Candidates in IC 348
The determination of the lower-end of the initial mass function (IMF)
provides strong constraints on star formation theories. We report here on a
search for isolated planetary-mass objects in the 3 Myr-old star-forming region
IC 348. Deep, narrowband CH4off and CH4on images were obtained with CFHT/WIRCam
over 0.11 sq.deg. in the central part of IC 348 to identify young T-dwarfs from
their 1.6 um methane absorption bands. We report three faint T-dwarf candidates
with CH4on-CH4off colours > 0.4 mag. Extinction was estimated for each
candidate and lies in the range Av ~ 5-12 mag. Comparisons with T-dwarf
spectral models, and colour/colour and colour/magnitude diagrams, reject two of
the three candidates because of their extreme z' - J blueness. The one
remaining object is not thought to be a foreground field dwarf because of a
number density argument and also its strong extinction Av - 12 mag, or thought
to be a background field T-dwarf which would be expected to be much fainter.
Models and diagrams give this object a preliminary T6 spectral type. With a few
Jupiter masses, the young T-dwarf candidate reported here is potentially
amongst the youngest, lowest mass objects detected in a star-forming region so
far. Its frequency is consistent with the extrapolation of current lognormal
IMF estimates down to the planetary mass domain.Comment: 10 pages, 10 figures, 5 tables. Accepted for publication in A&A,
21/08/2009. Update: two references made more recent, inclusion of one extra
reference. 16/09/200
Testing grain surface chemistry : a survey of deuterated formaldehyde and methanol in low-mass Class 0 protostars
Context : Despite the low cosmic abundance of deuterium (D/H ~ 1e-5), large
degrees of deuterium fractionation in molecules are observed in star forming
regions with enhancements that can reach 13 orders of magnitude, which current
models have difficulties to account for.
Aims : Multi-isotopologue observations are a very powerful constraint for
chemical models. The aim of our observations is to understand the processes
forming the observed large abundances of methanol and formaldehyde in low-mass
protostellar envelopes (gas-phase processes ? chemistry on the grain surfaces
?) and better constrain the chemical models. Methods : Using the IRAM 30m
single-dish telescope, we observed deuterated formaldehyde (HDCO and D2CO) and
methanol (CH2DOH, CH3OD, and CHD2OH) towards a sample of seven low-mass class 0
protostars. Using population diagrams, we then derive the fractionation ratios
of these species (abundance ratio between the deuterated molecule and its main
isotopologue) and compare them to the predictions of grain chemistry models.
Results : These protostars show a similar level of deuteration as in
IRAS16293-2422, where doubly-deuterated methanol -- and even triply-deuterated
methanol -- were first detected. Our observations point to the formation of
methanol on the grain surfaces, while formaldehyde formation cannot be fully
pined down. While none of the scenarii can be excluded (gas-phase or grain
chemistry formation), they both seem to require abstraction reactions to
reproduce the observed fractionations.Comment: 21 pages, 12 figures, accepted by A&