310 research outputs found
The 'Forbidden' Abundance of Oxygen in the Sun
We reexamine closely the solar photospheric line at 6300 A, which is
attributed to a forbidden line of neutral oxygen, and is widely used in
analyses of other late-type stars.
We use a three-dimensional time-dependent hydrodynamical model solar
atmosphere which has been tested successfully against observed granulation
patterns and an array of absorption lines. We show that the solar line is a
blend with a Ni I line, as previously suggested but oftentimes neglected.
Thanks to accurate atomic data on the [O I] and Ni I lines we are able to
derive an accurate oxygen abundance for the Sun: log epsilon (O) = 8.69 +/-
0.05 dex, a value at the lower end of the distribution of previously published
abundances, but in good agreement with estimates for the local interstellar
medium and hot stars in the solar neighborhood. We conclude by discussing the
implication of the Ni I blend on oxygen abundances derived from the [O I] 6300
A line in disk and halo stars.Comment: 16 pages, 3 eps figures included; a more compact PostScript version
created using emulateapj.sty is available from
http://hebe.as.utexas.edu/recent_publi.html; to appear in ApJ
Determining the Physical Properties of the B Stars I. Methodology and First Results
We describe a new approach to fitting the UV-to-optical spectra of B stars to
model atmospheres and present initial results. Using a sample of lightly
reddened stars, we demonstrate that the Kurucz model atmospheres can produce
excellent fits to either combined low dispersion IUE and optical photometry or
HST FOS spectrophotometry, as long as the following conditions are fulfilled:
1) an extended grid of Kurucz models is employed,
2) the IUE NEWSIPS data are placed on the FOS absolute flux system using the
Massa & Fitzpatrick (1999) transformation, and
3) all of the model parameters and the effects of interstellar extinction are
solved for simultaneously.
When these steps are taken, the temperatures, gravities, abundances and
microturbulence velocities of lightly reddened B0-A0 V stars are determined to
high precision. We also demonstrate that the same procedure can be used to fit
the energy distributions of stars which are reddened by any UV extinction curve
which can be expressed by the Fitzpatrick & Massa (1990) parameterization
scheme.
We present an initial set of results and verify our approach through
comparisons with angular diameter measurements and the parameters derived for
an eclipsing B star binary. We demonstrate that the metallicity derived from
the ATLAS 9 fits to main sequence B stars is essentially the Fe abundance. We
find that a near zero microturbulence velocity provides the best-fit to all but
the hottest or most luminous stars (where it may become a surrogate for
atmospheric expansion), and that the use of white dwarfs to calibrate UV
spectrophotometry is valid.Comment: 17 pages, including 2 pages of Tables and 6 pages of Figures.
Astrophysical Jounral, in pres
Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters
As a first step for studying star formation in the extreme outer Galaxy
(EOG), we obtained deep near-infrared images of two embedded clusters at the
northern and southern CO peaks of Cloud 2, which is one of the most distant
star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With
high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the
IRCS imager at the Subaru telescope, we detected cluster members with a mass
detection limit of < 0.1 M_{sun}, which is well into the substellar regime.
These high quality data enables a comparison of EOG to those in the solar
neighborhood on the same basis for the first time. Before interpreting the
photometric result, we have first constructed the NIR color-color diagram
(dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the
Mauna Kea Observatory filter system and also for the low metallicity
environment since the metallicity in EOG is much lower than those in the solar
neighborhood. The estimated stellar density suggests that an ``isolated type''
star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation
is ongoing in Cloud 2-S. Despite the difference of the star formation mode,
other characteristics of the two clusters are found to be almost identical: (1)
K-band luminosity function (KLF) of the two clusters are quite similar, as is
the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the
estimated star formation efficiencies (SFEs) for both clusters are typical
compared to those of embedded clusters in the solar neighborhood (~ 10 %). The
similarity of two independent clusters with a large separation (~ 25 pc)
strongly suggest that their star formation activities were triggered by the
same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap
Interstellar Turbulence II: Implications and Effects
Interstellar turbulence has implications for the dispersal and mixing of the
elements, cloud chemistry, cosmic ray scattering, and radio wave propagation
through the ionized medium. This review discusses the observations and theory
of these effects. Metallicity fluctuations are summarized, and the theory of
turbulent transport of passive tracers is reviewed. Modeling methods, turbulent
concentration of dust grains, and the turbulent washout of radial abundance
gradients are discussed. Interstellar chemistry is affected by turbulent
transport of various species between environments with different physical
properties and by turbulent heating in shocks, vortical dissipation regions,
and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered
and accelerated in turbulent magnetic waves and shocks, and they generate
turbulence on the scale of their gyroradii. Radio wave scintillation is an
important diagnostic for small scale turbulence in the ionized medium, giving
information about the power spectrum and amplitude of fluctuations. The theory
of diffraction and refraction is reviewed, as are the main observations and
scintillation regions.Comment: 46 pages, 2 figures, submitted to Annual Reviews of Astronomy and
Astrophysic
Si and Fe depletion in Galactic star-forming regions observed by the Spitzer Space Telescope
We report the results of the mid-infrared spectroscopy of 14 Galactic
star-forming regions with the high-resolution modules of the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope. We detected [SiII]
35um, [FeII] 26um, and [FeIII] 23um as well as [SIII] 33um and H2 S(0) 28um
emission lines. Using the intensity of [NII] 122um or 205um and [OI] 146um or
63um reported by previous observations in four regions, we derived the ionic
abundance Si+/N+ and Fe+/N+ in the ionized gas and Si+/O0 and Fe+/O0 in the
photodissociation gas. For all the targets, we derived the ionic abundance of
Si+/S2+ and Fe2+/S2+ for the ionized gas. Based on photodissociation and HII
region models the gas-phase Si and Fe abundance are suggested to be 3-100% and
<8% of the solar abundance, respectively, for the ionized gas and 16-100% and
2-22% of the solar abundance, respectively, for the photodissociation region
gas. Since the [FeII] 26um and [FeIII] 23um emissions are weak, the high
sensitivity of the IRS enables to derive the gas-phase Fe abundance widely in
star-forming regions. The derived gas-phase Si abundance is much larger than
that in cool interstellar clouds and that of Fe. The present study indicates
that 3-100% of Si atoms and <22% of Fe atoms are included in dust grains which
are destroyed easily in HII regions, probably by the UV radiation. We discuss
possible mechanisms to account for the observed trend; mantles which are
photodesorbed by UV photons, organometallic complexes, or small grains.Comment: 43 pages with 7 figures, accepted in Astrophysical Journa
The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822
To test the existence of a possible radial gradient in oxygen abundances
within the Local Group dwarf irregular galaxy NGC 6822, we have obtained
optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6-m
telescope at ESO La Silla. The extent of the measured nebulae spans
galactocentric radii in the range between 0.05 kpc and 2 kpc (over four
exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kalpha,
Kbeta, KD28e), the temperature-sensitive [O III] 4363 emission line was
detected, and direct oxygen abundances were derived. Oxygen abundances for the
remaining H II regions were derived using bright-line methods. The oxygen
abundances for three A-type supergiant stars are slightly higher than nebular
values at comparable radii. Linear least-square fits to various subsets of
abundance data were obtained. When all of the measured nebulae are included, no
clear signature is found for an abundance gradient. A fit to only newly
observed H II regions with [O III] 4363 detections yields an oxygen abundance
gradient of -0.14 +/- 0.07 dex/kpc. The gradient becomes slightly more
significant (-0.16 +/- 0.05 dex/kpc) when three additional H II regions with [O
III] 4363 measurements from the literature are added. Assuming no abundance
gradient, we derive a mean nebular oxygen abundance 12+log(O/H) = 8.11 +/- 0.10
from [O III] 4363 detections in the five H II regions from our present data;
this mean value corresponds to [O/H] = -0.55.Comment: Accepted, Ap.J.; 25 pages (AASTeX 5.2; emulateapj) with 14 figures.
Full paper with color figures may be retrieved from
http://www.astro.umn.edu/~hlee
Images IV: Strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies since z=0.8
Intermediate mass galaxies (logM(Msun)>10) at z~0.6 are the likeliest
progenitors of the present-day numerous population of spirals. There is growing
evidence that they have evolved rapidly since the last 6 to 8 Gyr ago, and
likely have formed a significant fraction of their stellar mass, often showing
perturbed morphologies and kinematics. We have gathered a representative sample
of 88 such galaxies and have provided robust estimates of their gas phase
metallicity. For doing so, we have used moderate spectral resolution
spectroscopy at VLT/FORS2 with unprecedented high S/N allowing to remove biases
coming from interstellar absorption lines and extinction to establish robust
values of R23=([OII]3727 + [OIII]4959,5007)/Hbeta. We definitively confirm that
the predominant population of z~0.6 starbursts and luminous IR galaxies (LIRGs)
are on average, two times less metal rich than the local galaxies at a given
stellar mass. We do find that the metal abundance of the gaseous phase of
galaxies is evolving linearly with time, from z=1 to z=0 and after comparing
with other studies, from z=3 to z=0. Combining our results with the reported
evolution of the Tully Fisher relation, we do find that such an evolution
requires that ~30% of the stellar mass of local galaxies have been formed
through an external supply of gas, thus excluding the close box model. Distant
starbursts & LIRGs have properties (metal abundance, star formation efficiency
& morphologies) similar to those of local LIRGs. Their underlying physics is
likely dominated by gas infall probably through merging or interactions. Our
study further supports the rapid evolution of z~0.4-1 galaxies. Gas exchanges
between galaxies is likely the main cause of this evolution.Comment: 21 pages, 12 figures, A&A, In pres
The High Velocity Gas toward Messier 5: Tracing Feedback Flows in the Inner Galaxy
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph (STIS E140M) observations of the post-asymptotic giant
branch star ZNG 1 in the globular cluster Messier 5 (l=3.9, b=+47.7; d=7.5 kpc,
z=+5.3 kpc). High velocity absorption is seen in C IV, Si IV, O VI, and lower
ionization species at LSR velocities of -140 and -110 km/s. We conclude that
this gas is not circumstellar on the basis of photoionization models and path
length arguments. Thus, the high velocity gas along the ZNG 1 sight line is the
first evidence that highly-ionized HVCs can be found near the Galactic disk. We
measure the metallicity of these HVCs to be [O/H]=+0.22\pm0.10, the highest of
any known HVC. Given the clouds' metallicity and distance constraints, we
conclude that these HVCs have a Galactic origin. This sight line probes gas
toward the inner Galaxy, and we discuss the possibility that these HVCs may be
related to a Galactic nuclear wind or Galactic fountain circulation in the
inner regions of the Milky Way.Comment: 23 pages, 11 figures, 7 table
Abundances of the elements in the solar system
A review of the abundances and condensation temperatures of the elements and
their nuclides in the solar nebula and in chondritic meteorites. Abundances of
the elements in some neighboring stars are also discussed.Comment: 42 pages, 11 tables, 8 figures, chapter, In Landolt- B\"ornstein, New
Series, Vol. VI/4B, Chap. 4.4, J.E. Tr\"umper (ed.), Berlin, Heidelberg, New
York: Springer-Verlag, p. 560-63
Chemical Abundances and Dust in Planetary Nebulae in the Galactic Bulge
We present mid-infrared Spitzer spectra of eleven planetary nebulae in the
Galactic Bulge. We derive argon, neon, sulfur, and oxygen abundances for them
using mainly infrared line fluxes combined with some optical line fluxes from
the literature. Due to the high extinction toward the Bulge, the infrared
spectra allow us to determine abundances for certain elements more accurately
that previously possible with optical data alone. Abundances of argon and
sulfur (and in most cases neon and oxygen) in planetary nebulae in the Bulge
give the abundances of the interstellar medium at the time their progenitor
stars formed; thus these abundances give information about the formation and
evolution of the Bulge. The abundances of Bulge planetary nebulae tend to be
slightly higher than those in the Disk on average, but they do not follow the
trend of the Disk planetary nebulae, thus confirming the difference between
Bulge and Disk evolution. Additionally, the Bulge planetary nebulae show
peculiar dust properties compared to the Disk nebulae. Oxygen-rich dust feature
(crystalline silicates) dominate the spectra of all of the Bulge planetary
nebulae; such features are more scarce in Disk nebulae. Additionally,
carbon-rich dust features (polycyclic aromatic hydrocarbons) appear in roughly
half of the Bulge planetary nebulae in our sample, which is interesting in
light of the fact that this dual chemistry is comparatively rare in the Milky
Way as a whole.Comment: 16 pages, 5 figures, accepted to Ap
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