4,908 research outputs found
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
Evidence for Differential Rotation on a T Tauri Star
Five years of photometric monitoring of the T Tauri star HBC 338 in NGC 1333
has revealed that it is a periodic variable, but the period has changed
significantly with time. From 2000-2003, a period near 5.6 days was observed,
while in the last two seasons, the dominant period is near 4.6 days. No other T
Tauri star has been seen to change its period by such a large percentage. We
propose a model in which a differentially rotating star is seen nearly
equator-on and a high latitude spot has gradually been replaced by a low
latitude spot. We show that this model provides an excellent fit to the
observed shapes of the light curves at each epoch. The amplitude and sense of
the inferred differential rotation is similar to what is seen on the Sun. This
may be surprising given the likely high degree of magnetic surface activity on
the star relative to the Sun but we note that HBC 338 is clearly an exceptional
T Tauri star.Comment: Acepted for publication in PAS
Methanol in the sky with diamonds
The present of gas phase methanol in dense interstellar molecular clouds was established by radio detection of its rotational emission lines. However, the position, width, and profile of a absorption band near 1470 cm(exp -1) in the IR spectra of many dense molecular clouds strongly suggests that solid methanol is an important component of interstellar ices. In an attempt to better constrain the identification of 1470 cm(exp -1) feature, we began a program to search for other characteristic absorption bands of solid state methanol in the spectra of objects known to produce this band. One such feature is now identified in the spectra of several dense molecular clouds and its position, width, and profile fit well with those of laboratory H2O:CH3OH ices. Thus, the presence of methanol-bearing ices in space is confirmed
Global phylogeography and evolution of chelonid fibropapilloma-associated herpesvirus
A global phylogeny for chelonid fibropapilloma-associated herpesvirus (CFPHV), the most likely aetiological agent of fibropapillomatosis (FP) in sea turtles, was inferred, using dated sequences, through Bayesian Markov chain Monte Carlo analysis and used to estimate the virus evolutionary rate independent of the evolution of the host, and to resolve the phylogenetic positions of new haplotypes from Puerto Rico and the Gulf of Guinea. Four phylogeographical groups were identified: eastern Pacific, western Atlantic/eastern Caribbean, mid-west Pacific and Atlantic. The latter comprises the Gulf of Guinea and Puerto Rico, suggesting recent virus gene flow between these two regions. One virus haplotype from Florida remained elusive, representing either an independent lineage sharing a common ancestor with all other identified virus variants or an Atlantic representative of the lineage giving rise to the eastern Pacific group. The virus evolutionary rate ranged from 1.62x10(-4) to 2.22x10(-4) substitutions per site per year, which is much faster than what is expected for a herpesvirus. The mean time for the most recent common ancestor of the modern virus variants was estimated at 192.90-429.71 years ago, which, although more recent than previous estimates, still supports an interpretation that the global FP pandemic is not the result of a recent acquisition of a virulence mutation(s). The phylogeographical pattern obtained seems partially to reflect sea turtle movements, whereas altered environments appear to be implicated in current FP outbreaks and in the modern evolutionary history of CFPHV.DNER-PR; US NMFS (NMFS-NOAA) [NA08NMF4720436]; US-Fish and Wildlife Service (USFWS); Sociedad Chelonia; WIDECAST; US Environmental Protection Agency (US-EPA); Lisbon Oceanarium, Portugal; Interdisciplinary Research Center for Animal Health of the Faculty of Veterinary Medicine of the Technical University of Lisbon (FMV/TUL)info:eu-repo/semantics/publishedVersio
Natural Coronagraphic Observations of the Eclipsing T Tauri System KH 15D: Evidence for Accretion and Bipolar Outflow in a WTTS
We present high resolution (R 44,000) UVES spectra of the eclipsing
pre-main sequence star KH 15D covering the wavelength range 4780 to 6810 {\AA}
obtained at three phases: out of eclipse, near minimum light and during egress.
The system evidently acts like a natural coronagraph, enhancing the contrast
relative to the continuum of hydrogen and forbidden emission lines during
eclipse. At maximum light the H equivalent width was 2 {\AA} and
the profile showed broad wings and a deep central absorption. During egress the
equivalent width was much higher (70 {\AA}) and the broad wings, which
extend to 300 km/s, were prominent. During eclipse totality the
equivalent width was less than during egress (40 {\AA}) and the high
velocity wings were much weaker. H showed a somewhat different behavior,
revealing only the blue-shifted portion of the high velocity component during
eclipse and egress. [OI] 6300, 6363 lines are easily seen both
out of eclipse and when the photosphere is obscured and exhibit little or no
flux variation with eclipse phase. Our interpretation is that KH 15D, although
clearly a weak-line T Tauri star by the usual criteria, is still accreting
matter from a circumstellar disk, and has a well-collimated bipolar jet. As the
knife-edge of the occulting matter passes across the close stellar environment
it is evidently revealing structure in the magnetosphere of this pre-main
sequence star with unprecedented spatial resolution. We also show that there is
only a small, perhaps marginally significant, change in the velocity of the K7
star between the maximum light and egress phases probed here
Detection of doubly-deuterated methanol in the solar-type protostar IRAS16293-2422
We report the first detection of doubly-deuterated methanol (CHD2OH), as well
as firm detections of the two singly-deuterated isotopomers of methanol (CH2DOH
and CH3OD), towards the solar-type protostar IRAS16293-2422. From the present
multifrequency observations, we derive the following abundance ratios:
[CHD2OH]/[CH3OH] = 0.2 +/- 0.1, [CH2DOH]/[CH3OH] = 0.9 +/- 0.3, [CH3OD]/[CH3OH]
= 0.04 +/- 0.02. The total abundance of the deuterated forms of methanol is
greater than that of its normal hydrogenated counterpart in the circumstellar
material of IRAS16293-2422, a circumstance not previously encountered.
Formaldehyde, which is thought to be the chemical precursor of methanol,
possesses a much lower fraction of deuterated isotopomers (~ 20%) with respect
to the main isotopic form in IRAS16293-2422. The observed fractionation of
methanol and formaldehyde provides a severe challenge to both gas-phase and
grain-surface models of deuteration. Two examples of the latter model are
roughly in agreement with our observations of CHD2OH and CH2DOH if the
accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase
model predicts such a high atomic D/H ratio, and hence some key ingredient
seems to be missing.Comment: 5 pages, 3 figure
On the information content of incubation studies
The measurement of the production of CO2 from soils in incubation studies has been used for many years to gain information about the influence of different soils types, changing temperatures and water contents, as well as the addition of amendments on the soil respiration. While in the early years the kinetic modelling (or fitting) was restricted to the single or one pool model due to the possibility of solving the problem by log-transforming the observed data an using a linear regression for the estimation of the rate constant (by doing so an analytical solution can be applied), more recent publications chose multi-pool models (2, 3, and even 4-pools), which can will be fitted iteratively using appropriate computer software. In general, there are different methods used in literature to estimate the kinetic parameters resulting in different kinetic parameter values even for the same data set. Additionally, screening of existing literature revealed that the 2-pool model (or even higher pool models) were sometimes obviously wrong fitted or over fitted. In our presentation, we will show how different constrains in the fitting process will influence the results of the kinetic parameter values, how obviously wrong fitting and overfitting can be easily detected, and how the information content of the incubation data can be easily judged prior any fitting. Finally, we will provide recommendations how to extract information from incubation experiments
Herschel observations of EXtra-Ordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey Toward Orion KL II. Chemical Implications
We present chemical implications arising from spectral models fit to the
Herschel/HIFI spectral survey toward the Orion Kleinmann-Low nebula (Orion KL).
We focus our discussion on the eight complex organics detected within the HIFI
survey utilizing a novel technique to identify those molecules emitting in the
hottest gas. In particular, we find the complex nitrogen bearing species
CHCN, CHCN, CHCN, and NHCHO systematically
trace hotter gas than the oxygen bearing organics CHOH, CHOH,
CHOCH, and CHOCHO, which do not contain nitrogen. If these
complex species form predominantly on grain surfaces, this may indicate
N-bearing organics are more difficult to remove from grain surfaces than
O-bearing species. Another possibility is that hot (T300 K)
gas phase chemistry naturally produces higher complex cyanide abundances while
suppressing the formation of O-bearing complex organics. We compare our derived
rotation temperatures and molecular abundances to chemical models, which
include gas-phase and grain surface pathways. Abundances for a majority of the
detected complex organics can be reproduced over timescales 10
years, with several species being under predicted by less than 3.
Derived rotation temperatures for most organics, furthermore, agree reasonably
well with the predicted temperatures at peak abundance. We also find that
sulfur bearing molecules which also contain oxygen (i.e. SO, SO, and OCS)
tend to probe the hottest gas toward Orion KL indicating the formation pathways
for these species are most efficient at high temperatures.Comment: 31 pages, 6 figures, 1 Table, accepted to the Astrophysical Journa
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