6,683 research outputs found
Do large rate coefficients for ion-polar neutral reactions have a serious effect on chemical models of dense clouds?
In order to incorporate large ion-polar neutral rate coefficients into existing gas phase reaction networks, it is necessary to utilize simplified theoretical treatments because of the significant number of rate coefficients needed. The authors have used two simple theoretical treatments: the locked dipole approach of Moran and Hamill for linear polar neutrals and the trajectory scaling approach of Su and Chesnavich for nonlinear polar neutrals. The former approach is suitable for linear species because in the interstellar medium these are rotationally relaxed to a large extent and the incoming charged reactants can lock their dipoles into the lowest energy configuration. The latter approach is a better approximation for nonlinear neutral species, in which rotational relaxation is normally less severe and the incoming charged reactants are not as effective at locking the dipoles. The treatments are in reasonable agreement with more detailed long range theories and predict an inverse square root dependence on kinetic temperature for the rate coefficient. Compared with the locked dipole method, the trajectory scaling approach results in rate coefficients smaller by a factor of approximately 2.5
Observations of pre-stellar cores
Our understanding of the physical and chemical structure of pre-stellar
cores, the simplest star-forming sites, has significantly improved since the
last IAU Symposium on Astrochemistry (South Korea, 1999). Research done over
these years has revealed that major molecular species like CO and CS
systematically deplete onto dust grains at the interior of pre-stellar cores,
while species like N2H+ and NH3 survive in the gas phase and can usually be
detected towards the core centers. Such a selective behaviour of molecular
species gives rise to a differentiated (onion-like) chemical composition, and
manifests itself in molecular maps as a dichotomy between centrally peaked and
ring-shaped distributions. From the point of view of star-formation studies,
the identification of molecular inhomogeneities in cores helps to resolve past
discrepancies between observations made using different tracers, and brings the
possibility of self-consistent modelling of the core internal structure. Here I
present recent work on determining the physical and chemical structure of two
pre-stellar cores, L1498 and L1517B, using observations in a large number of
molecules and Monte Carlo radiative transfer analysis. These two cores are
typical examples of the pre-stellar core population, and their chemical
composition is characterized by the presence of large freeze out holes in most
molecular species. In contrast with these chemically processed objects, a new
population of chemically young cores has started to emerge. The characteristics
of its most extreme representative, L1521E, are briefly reviewed.Comment: 10 pages, 5 figures. To appear in IAU 231 conf. proc.
"Astrochemistry: Recent Successes and Current Challenges," eds. D.C. Lis,
G.A. Blake, and E. Herbs
Laboratory millimeter and submillimeter spectrum of HOC^+
The J = 1â2, 2â3, and 3â4 rotational transitions of the molecular ion HOC^+ have been measured in the laboratory at frequencies from 178 to 358 GHz. The data should permit astronomers to confirm the recent possible sighting of the J = 1â0 transition of HOC^+ in Sgr B2 at 89.5 GHz
Rotation in the Orion Nebula Cluster
Eighteen fields in the Orion Nebula Cluster (ONC) have been monitored for one
or more observing seasons from 1990-99 with a 0.6-m telescope at Wesleyan
University. Photometric data were obtained in Cousins I on 25-40 nights per
season. Results from the first 3 years of monitoring were analyzed by Choi &
Herbst (1996; CH). Here we provide an update based on 6 more years of
observation and the extensive optical and IR study of the ONC by Hillenbrand
(1997) and Hillenbrand et al. (1998). Rotation periods are now available for
134 ONC members. Of these, 67 were detected at multiple epochs with identical
periods by us and 15 more were confirmed by Stassun et al. (1999) in their
study of Ori OBIc/d. The bimodal period distribution for the ONC is confirmed,
but we also find a clear dependence of rotation period on mass. This can be
understood as an effect of deuterium burning, which temporarily slows the
contraction and thus spin-up of stars with M <0.25 solar masses and ages of ~1
My. Stars with M <0.25 solar masses have not had time to bridge the gap in the
period distribution at ~4 days. Excess H-K and I-K emission, as well as CaII
infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but
significant correlations with rotation period among stars with M >0.25 solar
masses. Our results provide new observational support for the importance of
disks in the early rotational evolution of low mass stars. [abridged]Comment: 18 pages of text, 17 figures, and 4 tables; accepted for publication
in The Astronomical Journa
The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster
We have determined new rotation periods for 404 stars in the Orion Nebula
Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on
La Silla, Chile. Mass estimates are available for 335 of these and most have M
< 0.3 M_sun. We confirm the existence of a bimodal period distribution for the
higher mass stars in our sample and show that the median rotation rate
decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While
the spread in angular momentum (J) at any given mass is more than a factor of
10, the majority of lower mass stars in the ONC rotate at rates approaching 30%
of their critical break-up velocity, as opposed to 5-10% for solar-like stars.
This is a consequence of both a small increase in observed specific angular
momentum (j=J/M) and a larger decrease in the critical value of j with
decreasing mass. Perhaps the most striking fact, however, is that j varies by
so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The
distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in
nature to what is found in the Pleiades (age ~ 100 My). These observations
provide a significant new guide and test for models of stellar angular momentum
evolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure
X-ray Emission from the Weak-lined T Tauri Binary System KH 15D
The unique eclipsing, weak-lined T Tauri star KH 15D has been detected as an
X-ray source in a 95.7 ks exposure from the Chandra X-ray Observatory archives.
A maximum X-ray luminosity of 1.5 x 10^{29} erg s is derived in the
0.5--8 keV band, corresponding to L_{X}/L_bol = 7.5 x 10^{-5}. Comparison with
samples of stars of similar effective temperature in NGC 2264 and in the Orion
Nebula Cluster shows that this is about an order of magnitude low for a typical
star of its mass and age. We argue that the relatively low luminosity cannot be
attributed to absorption along the line of sight but implies a real deficiency
in X-ray production. Possible causes for this are considered in the context of
a recently proposed eccentric binary model for KH 15D. In particular, we note
that the visible component rotates rather slowly for a weak-lined T Tauri star
and has possibly been pseudosynchronized by tidal interaction with the primary
near periastron
The millimeter and submillimeter laboratory spectrum of methyl formate in its ground symmetric torsional state
Over 200 rotational lines of methyl formate in its ground (v-t = 0), symmetric (A) torsional state have been measured in the frequency range 140-550 GHz. Analysis of these and lower frequency transitions permits accurate prediction (â€0.1 MHz) of over 10,000 transitions at frequencies below 600 GHz with angular momentum J †50. The measured spectral lines have permitted identification of over 100 new methyl formate lines in Orion
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