16 research outputs found
The [O III] Veil: Astropause of Eta Carinae's Wind?
We present narrowband images of eta Carinae in the light of [O III] 5007
obtained with HST/WFPC2, as well as a ground-based image in the same emission
line with a larger field of view. These images show a thin veil of [O III]
emission around eta Car and its ejecta, confirming the existence of an
oxygen-bearing ``cocoon'' inferred from spectra. This [O III] veil may be the
remnant of the pre-outburst wind of eta Car, and its outer edge probably marks
the interface where eta Car's ejecta meet the stellar wind of the nearby O4
V((f)) star HD303308 or other ambient material -- i.e., it marks the
``astropause'' in eta Car's wind. This veil is part of a more extensive [O III]
shell that appears to be shaped and ionized by HD303308. A pair of HST images
with a 10 yr baseline shows no proper motion, limiting the expansion speed away
from eta Car to 12pm13 km/s, or an expansion age of a few times 10^4 yr. Thus,
this is probably the decelerated pre-outburst LBV wind of eta Car. The [O III]
morphology is very different from that seen in [N II], which traces young knots
of CNO-processed material; this represents a dramatic shift in the chemical
makeup of material recently ejected by eta Car. This change in the chemical
abundances may have resulted from the sudden removal of the star's outer
envelope during eta Car's 19th century outburst or an earlier but similar
event.Comment: 11 pages, 4 figs. Figs 1 and 3 in color. Accepted to AJ, October 200
Spectral Modelling of Star-Forming Regions in the Ultraviolet: Stellar Metallicity Diagnostics for High Redshift Galaxies
The chemical composition of high redshift galaxies is an important property
which gives clues to their past history and future evolution and yet is
difficult to measure with current techniques. In this paper we investigate new
metallicity indicators, based upon the strengths of stellar photospheric
features at rest-frame ultraviolet wavelengths. By combining the evolutionary
spectral synthesis code Starburst99 with the output from the non-LTE model
atmosphere code WM-basic, we have developed a code that can model the
integrated ultraviolet stellar spectra of star-forming regions at metallicities
between 1/20 and twice solar. We use our models to explore a number of spectral
regions that are sensitive to metallicity and clean of other spectral features.
The most promising metallicity indicator is an absorption feature between 1935
A and 2020 A, which arises from the blending of numerous Fe III transitions. We
compare our model spectra to observations of two well studied high redshift
star-forming galaxies, MS1512-cB58 (a Lyman break galaxy at z = 2.7276), and
Q1307-BM1163 (a UV-bright galaxy at z = 1.411). The profiles of the
photospheric absorption features observed in these galaxies are well reproduced
by the models. In addition, the metallicities inferred from their equivalent
widths are in good agreement with previous determinations based on interstellar
absorption and nebular emission lines. Our new technique appears to be a
promising alternative, or complement, to established methods which have only a
limited applicability at high redshifts.Comment: 18 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds
We have obtained HST and ground-based observations of a sample of 20 O-type
stars in the LMC and SMC, including six of the hottest massive stars known
(subtypes O2-3) in the R136 cluster. In general, these data include (a) the HST
UV spectra in order to measure the terminal velocities of the stellar winds,
(b) high signal-to-noise, blue-optical data where the primary temperature- and
gravity-sensitive photospheric lines are found, and (c) nebular-free H-alpha
profiles, which provide the mass-loss rates. The line-blanketed non-LTE
atmosphere code FASTWIND was then used to determine the physical parameters of
this sample of stars. We find good agreement between the synthetic line
profiles for the hydrogen, He I, and He II lines in the majority of the stars
we analyzed; the three exceptions show evidence of being incipiently resolved
spectroscopic binaries or otherwise spectral composites. One such system is
apparently an O3 V+O3 V eclipsing binary, and a follow-up radial velocity study
is planned to obtain Keplerian masses. Although we did not use them to
constrain the fits, good agreement is also found for the He I
and He II lines in the near-UV, which we plan to exploit in
future studies. Our effective temperatures are compared to those recently
obtained by Repolust, Puls & Herrero for a sample of Galactic stars using the
same techniques. We find that the Magellanic Cloud sample is
3,000-4,000K hotter than their Galactic counterparts for the early
through mid-O's. These higher temperatures are the consequence of a decreased
importance of wind emission, wind blanketing, and metal-line blanketing at
lower metallicities.Comment: Accepted for publication in the Astrophysical Journal. A postscript
version with the figures embedded can be found at
ftp://ftp.lowell.edu/pub/massey/haw.p
The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars, and Results from the Complete Sample
We analyze the optical and UV spectra of an additional sample of 20
Magellanic Cloud O stars, and draw conclusions from the complete sample of 40
stars. We find (1) The SMC O3-7 dwarfs are about 4000 K cooler than their
Galactic counterparts; this is in the sense expected from the decreased
signficiance of line-blanketing and wind-blanketing at lower metallicities. The
difference decreases with later types, becoming negligible by B0 V. A similar
difference is found for the supergiants. (2) The wind momentum of these stars
scales with luminosity and metallicty in the ways predicted by
radiatively-driven wind theory. (3) A comparison of the masses derived from
spectroscopy with those derived from stellar evolutionary theory shows a
significant discrepancy for stars hotter than 45000, although good agreement is
found for cooler stars. (4) For the hottest O stars (O2-3.5) neither the
NIII/NIV ratio, nor even the HeI/HeII ratio, does a good job of predicting the
effective temperature by itself. Instead, a full analysis is needed to derive
physical parameters. Thus there are O3.5V stars which are as hot or hotter than
stars classified as O2V. (5) The two stars with the most discordant radial
velocities in our sample happen to be O3 "field stars". This provides the first
compelling observational evidence that the "field" O stars in the Magellanic
Clouds may be runaway OB stars, ejected from their birth place.Comment: Accepted by the Astrophysical Journal. A version with
higher-resolution figures may be found at
ftp://ftp.lowell.edu/pub/massey/haw2final.pdf This replacement included a
revised version of Fig 29a and the accompanying tex
Quantitative Spectroscopy of O Stars at Low Metallicity. O Dwarfs in NGC 346
We present the results of a detailed UV and optical spectral analysis of the
properties of 6 dwarf O-type stars in the SMC H II region NGC 346. Stellar
parameters, chemical abundances, and wind parameters have been determined using
NLTE line blanketed models calculated with the photospheric code, Tlusty, and
with the wind code, CMFGEN. The results, in particular iron abundances,
obtained with the two NLTE codes compare very favorably, demonstrating that
basic photospheric parameters of O dwarfs can be reliably determined using NLTE
static model atmospheres. The two NLTE codes require a microturbulent velocity
to match the observed spectra. Our results hint at a decrease of the
microturbulent velocity from early O stars to late O stars. Similarly to
several recent studies of galactic, LMC and SMC stars, we derive effective
temperatures lower than predicted from the widely-used relation between
spectral type and Teff, resulting in lower stellar luminosities and lower
ionizing fluxes. From evolutionary tracks in the HR diagram, we find an age of
3 10^6 years for NGC 346. A majority of the stars in our sample reveal
CNO-cycle processed material at their surface during the MS stage, indicating
thus fast stellar rotation and/or very efficient mixing processes. We obtain an
overall metallicity, Z = 0.2 Zsun, in good agreement with other recent analyses
of SMC stars. The derived mass loss rate of the three most luminous stars
agrees with recent theoretical predictions. However, the three other stars of
our sample reveal very weak wind signatures. We obtain mass loss rates that are
significantly lower than 10^{-8} Msun/yr, which is below the predictions of
radiative line-driven wind theory by an order of magnitude or more. (abridged
version)Comment: 61 pages, 17 figures; to appear in ApJ, 595 (Oct 1, 2003); minor
revisions and addition
Nitrogen line spectroscopy in O-stars -- II. Surface nitrogen abundances for O-stars in the Large Magellanic Cloud
This is the second paper in a series aiming at the analysis of nitrogen
abundances in O-type stars, to enable further constraints on the early
evolution of massive stars. We investigate the NIV lambda4058 emission line
formation, provide nitrogen abundances for a substantial O-star sample in the
LMC, and compare our findings with recent predictions from stellar evolutionary
models. Stellar and wind parameters are determined by line profile fitting of
hydrogen, helium and nitrogen lines, based on synthetic spectra calculated by
FASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzing
all nitrogen lines present in the available optical spectra. The dominating
process responsible for emission at NIV lambda4058 in O-stars is the strong
depopulation of the lower level of the transition, which increases as a
function of Mdot. Unlike the NIII triplet emission, resonance lines do not play
a role for typical mass-loss rates and below. The bulk of our sample O-stars
seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and
helium enrichment is found. By comparing the nitrogen abundances as a function
of vsini ('Hunter-plot') with tailored evolutionary calculations, we identify a
considerable number of highly enriched objects at low rotation. Due to the low
initial abundance, the detection of strong Nitrogen enrichment in the bulk of
O-stars indicates that efficient mixing takes place already during the very
early phases of stellar evolution of LMC O-stars.Comment: Main paper: 23 pages, 9 figures. Appendix: 17 pages, 32 figures.
Accepted by Astronomy & Astrophysic