359 research outputs found
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti
The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a
substantial quantity of molecular gas while exhibiting dust properties similar
to those of a debris disk. We present resolved observations of the disk around
49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO
with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended
rotating structure viewed approximately edge-on and clear of detectable CO
emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum
emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk
structure and chemistry indicate that the inner disk is devoid of molecular
gas, while the outer gas disk between 40 and 200 AU from the star is dominated
by photochemistry from stellar and interstellar radiation. We determine
parameters for a model that reproduces the basic features of the spatially
resolved CO J=2-1 emission, the spectral energy distribution, and the
unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and
observable properties for a range of structural parameters. 49 Ceti appears to
be a rare example of a system in a late stage of transition between a gas-rich
protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap
The Faintness of the 158 um [CII] Transition in the z=6.42 Quasar SDSS J1148+5251
We report the non-detection of the [CII] 157.74 um transition in the z=6.42
quasar SDSS J1148+5251 after 37.5 hours of integration with the James Clerk
Maxwell Telescope. This transition is the main cooling line of the star-forming
interstellar medium, and usually the brightest FIR line in galaxies. Our
observed RMS of 1.3 mK in the Ta* scale translates to L([CII])<2.6 x 10^9 Lsun.
Using a recent estimate of the far-infrared continuum of this quasar, we derive
for SDSS J1148+5251 L([CII])/L(FIR)<5 x 10^-4, a ratio similar to that observed
in local ultra-luminous infrared galaxies but considerably smaller than what is
typical in nearby normal and starburst galaxies. This indicates that the small
L([CII])/L(FIR) ratio observed locally in luminous far-infrared objects also
persists at the highest redshifts.Comment: Five pages, one figure, accepted by the Astrophysical Journal Letter
The Effects of Doubly Ionized Chemistry on SH+ and S^+2 Abundances in X-ray Dominated Regions
Recent laboratory measurements for the S^+2 + H2 reaction find a total rate
coefficient significantly larger than previously used in theoretical models of
X-ray dominated regions (XDRs). While the branching ratio of the products is
unknown, one energetically possible route leads to the SH+ molecule, a known
XDR diagnostic. In this work, we study the effects of S^+2 on the formation of
SH+ and the destruction of S^+2 in XDRs. We find the predicted SH+ column
density for molecular gas surrounding an Active Galactic Nucleus (AGN)
increases by as much as 2 dex. As long as the branching ratio for S^+2 + H2 ->
SH+ + H+ exceeds a few percent, doubly ionized chemistry will be the dominant
pathway to SH+, which then initiates the formation of other sulfur-bearing
molecules. We also find that the high rate of S^+2 + H2 efficiently destroys
S^+2 once H2 forms, while the S^+2 abundance remains high in the atomic
hydrogen region. We discuss the possible consequences of S^+2 in the atomic
hydrogen region on mid-infrared diagnostics. The enhanced SH+ abundance has
important implications in the study of XDRs, while our conclusions for S^+2
could potentially impact the interpretation of Spitzer and SOFIA observations.Comment: 19 pages, 3 figures, Accepted for Publication in ApJ Letter
Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation
We analyze the dynamical expansion of the HII region, photodissociation
region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the
thermal and chemical processes in the time-dependent hydrodynamics code.
Following our previous paper, we investigate the time evolutions with various
ambient number densities and central stars. Our calculations show that basic
evolution is qualitatively similar among our models with different parameters.
The molecular gas is finally accumulated in the shell, and the gravitational
fragmentation of the shell is generally expected. The quantitative differences
among models are well understood with analytic scaling relations. The detailed
physical and chemical structure of the shell is mainly determined by the
incident FUV flux and the column density of the shell, which also follow the
scaling relations. The time of shell-fragmentation, and the mass of the
gathered molecular gas are sensitive tothe ambient number density. In the case
of the lower number density, the shell-fragmentation occurs over a longer
timescale, and the accumulated molecular gas is more massive. The variations
with different central stars are more moderate. The time of the
shell-fragmentation differs by a factor of several with the various stars of
M_* = 12-101 M_sun. According to our numerical results, we conclude that the
expanding HII region should be an efficient trigger for star formation in
molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap
Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the
physical properties of the molecular material in these reflection nebulae. Our
observations of NGC 2023 and NGC 7023 and the physical parameters we derive for
these nebulae are in good agreement with previous studies. Both NGC 1333 and
NGC 2068 have no previously-published analysis of near-infrared spectra. Our
study reveals that the rotational-vibrational states of molecular hydrogen in
NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We
determine that the relatively weak UV field illuminating NGC 1333 is the
primary cause of the difference. Further, we find that the density of the
emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4
cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC
7023 and NGC 2023. Our model fits to this nebula show that the bright,
H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to
what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023
show significant changes in both the near-infrared continuum and H_2 intensity
along the slit and offsets between the peaks of the H_2 and continuum emission.
We find that these brightness changes may correspond to real changes in the
density and temperatures of the emitting region, although uncertainties in the
total column of emitting material along a given line of sight complicates the
interpretation. The spatial difference in the peak of the H_2 and near-infrared
continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum
is due to a material which can survive closer to the star than H_2 can.Comment: Submitted for publication in ApJ. 34 pages including 12 embedded
postscript figures. Also available at
http://www.astronomy.ohio-state.edu/~martini/pub
Non-Gaussian Radio-Wave Scattering in the Interstellar Medium
It was recently suggested by Boldyrev & Gwinn that the characteristics of
radio scintillations from distant pulsars are best understood if the
interstellar electron-density fluctuations that cause the time broadening of
the radio pulses obey non-Gaussian statistics. In this picture the density
fluctuations are inferred to be strong on very small scales (). We argue that such density structures could correspond to the ionized
boundaries of molecular regions (clouds) and demonstrate that the power-law
distribution of scattering angles that is required to match the observations
arises naturally from the expected intersections of our line of sight with
randomly distributed, thin, approximately spherical ionized shells of this
type. We show that the observed change in the time-broadening behavior for
pulsar dispersion measures is consistent
with the expected effect of the general ISM turbulence, which should dominate
the scattering for nearby pulsars. We also point out that if the clouds are
ionized by nearby stars, then their boundaries may become turbulent on account
of an ionization front instability. This turbulence could be an alternative
cause of the inferred density structures. An additional effect that might
contribute to the strength of the small-scale fluctuations in this case is the
expected flattening of the turbulent density spectrum when the eddy sizes
approach the proton gyroscale.Comment: 15 pages, 3 figures, accepted to Ap
A quasi-time-dependent radiative transfer model of OH104.9+2.4
We investigate the pulsation-phase dependent properties of the circumstellar
dust shell (CDS) of the OH/IR star OH104.9+2.4 based on radiative transfer
modeling (RTM) using the code DUSTY. Our previous study concerning simultaneous
modeling of the spectral energy distribution (SED) and near-infrared (NIR)
visibilities (Riechers et al. 2004) has now been extended by means of a more
detailed analysis of the pulsation-phase dependence of the model parameters of
OH104.9+2.4. In order to investigate the temporal variation in the spatial
structure of the CDS, additional NIR speckle interferometric observations in
the K' band were carried out with the 6 m telescope of the Special
Astrophysical Observatory (SAO). At a wavelength of 2.12 micron the
diffraction-limited resolution of 74 mas was attained. Several key parameters
of our previous best-fitting model had to be adjusted in order to be consistent
with the newly extended amount of observational data. It was found that a
simple rescaling of the bolometric flux F_bol is not sufficient to take the
variability of the source into account, as the change in optical depth over a
full pulsation cycle is rather high. On the other hand, the impact of a change
in effective temperature T_eff on SED and visibility is rather small. However,
observations, as well as models for other AGB stars, show the necessity of
including a variation of T_eff with pulsation phase in the radiative transfer
models. Therefore, our new best-fitting model accounts for these changes.Comment: 7 pages, including 5 postscript figures and 3 tables. Published in
Astronomy and Astrophysics. (v1: accepted version; v2: published version,
minor grammatical changes
The Ionization Fraction in Dense Molecular Gas II: Massive Cores
We present an observational and theoretical study of the ionization fraction
in several massive cores located in regions that are currently forming stellar
clusters. Maps of the emission from the J = 1-> O transitions of C18O, DCO+,
N2H+, and H13CO+, as well as the J = 2 -> 1 and J = 3 -> 2 transitions of CS,
were obtained for each core. Core densities are determined via a large velocity
gradient analysis with values typically 10^5 cm^-3. With the use of
observations to constrain variables in the chemical calculations we derive
electron fractions for our overall sample of 5 cores directly associated with
star formation and 2 apparently starless cores. The electron abundances are
found to lie within a small range, -6.9 < log10(x_e) < -7.3, and are consistent
with previous work. We find no difference in the amount of ionization fraction
between cores with and without associated star formation activity, nor is any
difference found in electron abundances between the edge and center of the
emission region. Thus our models are in agreement with the standard picture of
cosmic rays as the primary source of ionization for molecular ions. With the
addition of previously determined electron abundances for low mass cores, and
even more massive cores associated with O and B clusters, we systematically
examine the ionization fraction as a function of star formation activity. This
analysis demonstrates that the most massive sources stand out as having the
lowest electron abundances (x_e < 10^-8).Comment: 35 pages (8 figures), using aaspp4.sty, to be published in
Astrophysical Journa
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