1,728 research outputs found
Debris Disk Radiative Transfer Simulation Tool (DDS)
A WWW interface for the simulation of spectral energy distributions of
optically thin dust configurations with an embedded radiative source is
presented. The density distribution, radiative source, and dust parameters can
be selected either from an internal database or defined by the user. This tool
is optimized for studying circumstellar debris disks where large grains are
expected to determine the far-infrared through millimeter dust reemission
spectral energy distribution. The tool is available at
http://aida28.mpia-hd.mpg.de/~swolf/ddsComment: Comp. Phys. Comm, 2005, in pres
An Optical Survey of the Partially Embedded Young Cluster in NGC 7129
NGC 7129 is a bright reflection nebula located in the molecular cloud complex
near l=105.4, b=+9.9, about 1.15 kpc distant. Embedded within the reflection
nebula is a young cluster dominated by a compact grouping of four early-type
stars: BD+65 1638 (B3V), BD+65 1637 (B3e), SVS 13 (B5e), and LkH-alpha 234
(B8e). About 80 H-alpha emission sources brighter than V~23 are identified in
the region, many of which are presumably T Tauri star members of the cluster.
We also present deep (V~23), optical (VRI) photometry of a field centered on
the reflection nebula and spectral types for more than 130 sources determined
from low dispersion, optical spectroscopy. The narrow pre-main sequence evident
in the color-magnitude diagram suggests that star formation was rapid and
coeval. A median age of about 1.8 Myr is inferred for the H-alpha and
literature-identified X-ray emission sources having established spectral types,
using pre-main sequence evolutionary models. Our interpretation of the
structure of the molecular cloud and the distribution of young stellar objects
is that BD+65 1638 is primarily responsible for evacuating the blister-like
cavity within the molecular cloud. LkH-alpha 234 and several embedded sources
evident in near infrared adaptive optics imaging have formed recently within
the ridge of compressed molecular gas. The compact cluster of low-mass stars
formed concurrently with the early-type members, concentrated within a central
radius of ~0.7 pc. Star formation is simultaneously occurring in a
semi-circular arc some ~3 pc in radius that outlines remaining dense regions of
molecular gas. High dispersion, optical spectra are presented for BD+65 1638,
BD+65 1637, SVS 13, LkH-alpha 234, and V350 Cep. These spectra are discussed in
the context of the circumstellar environments inferred for these stars.Comment: 45 pages, 20 figures, accepted for publication in the Astronomical
Journa
The spatial distribution of substellar objects in IC348 and the Orion Trapezium Cluster
Aims: Some theoretical scenarios suggest the formation of brown dwarfs as
ejected stellar embryos in star-forming clusters. Such a formation mechanism
can result in different spatial distributions of stars and substellar objects.
We aim to investigate the spatial structure of stellar and substellar objects
in two well sampled and nearby embedded clusters, namely IC348 and the Orion
Trapezium Cluster (OTC) to test this hypothesis. Methods:Deep near-infrared
K-band data complete enough to sample the substellar population in IC348 and
OTC are obtained from the literature. The spatial distribution of the K-band
point sources is analysed using the Minimum Spanning Tree (MST) method. The Q
parameter and the spanning trees are evaluated for stellar and substellar
objects as a function of cluster core radius R. Results: The stellar
population in both IC348 and OTC display a clustered distribution whereas the
substellar population is distributed homogeneously in space within twice the
cluster core radius. Although the substellar objects do not appear to be bound
by the cluster potential well, they are still within the limits of the cluster
and not significantly displaced from their birth sites. Conclusions: The
spatially homogeneous distribution of substellar objects is best explained by
assuming higher initial velocities, distributed in a random manner and going
through multiple interactions. The overall spatial coincidence of these objects
with the cluster locations can be understood if these objects are nevertheless
travelling slowly enough so as to feel the gravitational effect of the cluster.
The observations support the formation of substellar objects as ``ejected
stellar embryos''. Higher ejection velocities are necessary but net spatial
displacements may not be necessary to explain the observational data.Comment: 4 pages. Accepted by A&A Letter
An Enhanced Spectroscopic Census of the Orion Nebula Cluster
We report new spectral types or spectral classification constraints for over
600 stars in the Orion Nebula Cluster (ONC) based on medium resolution R~
1500-2000 red optical spectra acquired using the Palomar 200" and Kitt Peak
3.5m telescopes. Spectral types were initially estimated for F, G, and early K
stars from atomic line indices while for late K and M stars, constituting the
majority of our sample, indices involving TiO and VO bands were used. To ensure
proper classification, particularly for reddened, veiled, or
nebula-contaminated stars, all spectra were then visually examined for type
verification or refinement. We provide an updated spectral type table that
supersedes Hillenbrand (1997), increasing the percentage of optically visible
ONC stars with spectral type information from 68% to 90%. However, for many
objects, repeated observations have failed to yield spectral types primarily
due to the challenges of adequate sky subtraction against a bright and
spatially variable nebular background. The scatter between our new and our
previously determined spectral types is approximately 2 spectral sub-classes.
We also compare our grating spectroscopy results with classification based on
narrow-band TiO filter photometry from Da Rio et al. (2012, finding similar
scatter. While the challenges of working in the ONC may explain much of the
spread, we highlight several stars showing significant and unexplained bona
fide spectral variations in observations taken several years apart; these and
similar cases could be due to a combination of accretion and extinction
changes. Finally, nearly 20% of ONC stars exhibit obvious Ca II triplet
emission indicative of strong accretion.Comment: Accepted to the Astronomical Journal; 37 pages, including 11 Figures
and 3 Tables (one long table not reproduced here but available upon request
or from the journal
Continuous catalytic decomposition of methane
Water is conserved by employing sequence of reactions whereby 75 percent of methane from Sabatier reaction is decomposed to solid carbon and hydrogen; hydrogen is then separated from residual methane and utilized in usual Sabatier reaction to reduce remaining metabolic carbon dioxide
Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr
We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars
Periodic Eruptive Variability of the Isolated Pre-main-sequence Star V347 Aurigae
V347 Aurigae is associated with the small dark cloud L1438 and appears to be an isolated pre-main-sequence star located at distance d ≈ 200 pc. Multi-epoch, archival photometry reveals periodic brightness variations with amplitude V ≈ 2.0 mag occurring on timescales of ~160 days that have persisted for decades. Regular-cadence, optical imaging of the source with the Zwicky Transient Facility shows that a small reflection nebula illuminated by V347 Aur also fluctuates in brightness, at times fading completely. Multi-epoch, Keck/HIRES data suggests the presence of two distinct spectral components: a prominent emission-line-dominated spectrum with a heavily veiled continuum correlated with the bright photometric state, and an M-type absorption line spectrum associated with quiescence. All spectra exhibit strong Balmer and He i line emission, consistent with accretion, as well as high velocity emission arising from the forbidden transitions of [O i], [N ii], and [S ii] that are generally associated with collimated jets and disk winds. There is no evidence in existing high-dispersion spectroscopy or high-resolution imaging for binarity of V347 Aur. The repeating outburst events are possibly linked to accretion instabilities induced by an undetected companion or a structure within the circumstellar disk that periodically increases the mass accretion rate. V347 Aur is perhaps analogous to an EXor-type variable, though more regularly recurring
Periodic Eruptive Variability of the Isolated Pre-Main Sequence Star V347 Aurigae
V347 Aurigae is associated with the small dark cloud L1438 and appears to be
an isolated pre-main sequence star located at distance 200 pc.
Multi-epoch, archival photometry reveals periodic brightness variations with
amplitude magnitudes occurring on timescales of 160 days
that have persisted for decades. Regular cadence, optical imaging of the source
with the Zwicky Transient Facility shows that a small reflection nebula
illuminated by V347 Aur also fluctuates in brightness, at times fading
completely. Multi-epoch, Keck/HIRES data suggests the presence of two distinct
spectral components: a prominent emission-line dominated spectrum with a
heavily veiled continuum correlated with the bright photometric state, and an
M-type absorption line spectrum associated with quiescence. All spectra exhibit
strong Balmer and He I line emission, consistent with accretion, as well as
high velocity emission arising from the forbidden transitions of [O I], [N II],
and [S II] that are generally associated with collimated jets and disk winds.
There is no evidence in existing high dispersion spectroscopy or high
resolution imaging for binarity of V347 Aur. The repeating outburst events are
possibly linked to accretion instabilities induced by an undetected companion
or a structure within the circumstellar disk that periodically increases the
mass accretion rate. V347 Aur is perhaps analogous to an EXor-type variable,
though more regularly recurring.Comment: Accepted for publication in the Astronomical Journal; 17 pages, 16
figures, 1 tabl
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