58 research outputs found
The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity
We use the MARCS stellar atmosphere to derive the physical properties of 36
red supergiants (RSGs) in the LMC, and 39 RSGs in the SMC using
moderate-resolution optical spectrophotometry (4000-9000A) and broad-band
colors (V-R, V-K). The results from the dereddened V-R colors are in good
agreement with those derived from the spectrophotometry, but the dereddened V-K
colors give temperatures that are 3-4% warmer for the SMC data, with the LMC
and Milky Way showing a smaller but similar effect. We conclude that this
discrepancy is due to the limitations of 1D models. Our newly derived effective
temperatures and bolometric luminosities bring the Magellanic Cloud RSGs into
good agreement with stellar evolutionary models that include the effects of
rotation. A typical M2~I in the SMC is about 150 K cooler than its Galactic
counterpart; one in the LMC is about 50 K cooler. This is in the sense expected
due to the lower chemical abundances in the SMC and LMC, although it is not
sufficient to explain the shift in average RSG spectral type seen between the
SMC, LMC, and Milky Way. Instead, that is due primarily to the change in
Hayashi limit with metallicity, as first proposed by Elias et al. (1985).
Finally, our study confirms that many RSGs in the Magellanic Clouds are
significantly more reddened than OB stars, consistent with our recent findings
for Galactic stars that circumstellar dust may contribute several magnitudes of
extra visual extinction.Comment: Accepted by the Astrophysical Journa
Perfusion-CT guided intravenous thrombolysis in patients with unknown-onset stroke: a randomized, double-blind, placebo-controlled, pilot feasibility trial
Introduction: Patients with unknown stroke onset are generally excluded from acute recanalisation treatments. We designed a pilot study to assess feasibility of a trial of perfusion computed tomography (PCT)-guided thrombolysis in patients with ischemic tissue at risk of infarction and unknown stroke onset. Methods: Patients with a supratentorial stroke of unknown onset in the middle cerebral artery territory and significant volume of at-risk tissue on PCT were randomized to intravenous thrombolysis with alteplase (0.9mg/kg) or placebo. Feasibility endpoints were randomization and blinded treatment of patients within 2h after hospital arrival, and the correct application (estimation) of the perfusion imaging criteria. Results: At baseline, there was a trend towards older age [69.5 (57-78) vs. 49 (44-78) years] in the thrombolysis group (n = 6) compared to placebo (n = 6). Regarding feasibility, hospital arrival to treatment delay was above the allowed 2h in three patients (25%). There were two protocol violations (17%) regarding PCT, both underestimating the predicted infarct in patients randomized in the placebo group. No symptomatic hemorrhage or death occurred during the first 7days. Three of the four (75%) and one of the five (20%) patients were recanalized in the thrombolysis and placebo group respectively. The volume of non-infarcted at-risk tissue was 84 (44-206) cm3 in the treatment arm and 29 (8-105) cm3 in the placebo arm. Conclusions: This pilot study shows that a randomized PCT-guided thrombolysis trial in patients with stroke of unknown onset may be feasible if issues such as treatment delays and reliable identification of tissue at risk of infarction tissue are resolved. Safety and efficiency of such an approach need to be establishe
The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds
We investigate the red supergiant (RSG) content of the SMC and LMC using
multi-object spectroscopy on a sample of red stars previously identified by
{\it BVR} CCD photometry. We obtained high accuracy ( km s) radial
velocities for 118 red stars seen towards the SMC and 167 red stars seen
towards the LMC, confirming most of these (89% and 95%, respectively) as red
supergiants (RSGs). Spectral types were also determined for most of these RSGs.
We find that the distribution of spectral types is skewed towards earlier type
at lower metallicities: the average (median) spectral type is K5-7 I in the
SMC, M1 I in the LMC, and M2 I in the Milky Way. We argue that RSGs in the
Magellanic Clouds are 100deg (LMC) and 300deg (SMC) cooler than Galactic RSGs
of the same spectral type. We compare the distribution of RSGs in the H-R
diagram to that of various stellar evolutionary models; we find that none of
the models produce RSGs as cool and luminous as what is actually observed. In
all of our H-R diagrams, however, there is an elegant sequence of decreasing
effective temperatures with increasing luminosities; explaining this will be an
important test of future stellar evolutionary models.Comment: Version with eps figures embedded can be obtained from
ftp://ftp.lowell.edu/pub/massey/rsgs.ps.gz Accepted by the Astronomical
Journa
World Association for the Advancement of Veterinary Parasitology (WAAVP) guideline: anthelmintic combination products targeting nematode infections of ruminants and horses
Increasing threats from anthelmintic resistant nematode populations warrant and motivate a reappraisal of chemotherapeutic strategies for nematode control in ruminant livestock and horses. The objective of this paper is to present a guideline for the evaluation of products containing two or more constituent anthelmintic actives in a single dosage form for the treatment of nematode infections in these animals. At present, regulatory policies on the approval of such products vary across jurisdictions, and this World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) guideline should enable the harmonization of the requirements. This guideline makes clear recommendations on the minimal standards needed, but stresses that registration dossiers for combination anthelmintic products submitted for approval must conform to the standards and practices already established in existing guidelines for anthelmintics
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
The Physical Properties of Red Supergiants: Comparing Theory and Observations
Red supergiants (RSGs) are an evolved stage in the life of intermediate
massive stars (than than 25 solar masses). For many years, their location in
the H-R diagram was at variance with the evolutionary models. Using the MARCS
stellar atmospheres, we have determined new effective temperatures and
bolometric luminosities for RSGs in the Milky Way, LMC, and SMC, and our work
has resulted in much better agreement with the evolutionary models. We have
also found evidence of significant visual extinction due to circumstellar dust.
Although in the Milky Way the RSGs contribute only a small fraction (than than
1 percent) of the dust to the interstellar medium (ISM), in starburst galaxies
or galaxies at large look-back times, we expect that RSGs may be the main dust
source. We are in the process of extending this work now to RSGs of higher and
lower metallicities using the galaxies M31 and WLM.Comment: Invited review; to appear in Massive Stars as Cosmic Engines, IAU
Symp. 250, ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University
Press
Yellow Supergiants in the Small Magellanic Cloud (SMC): Putting Current Evolutionary Theory to the Test
The yellow supergiant content of nearby galaxies provides a critical test of
massive star evolutionary theory. While these stars are the brightest in a
galaxy, they are difficult to identify because a large number of foreground
Milky Way stars have similar colors and magnitudes. We previously conducted a
census of yellow supergiants within M31 and found that the evolutionary tracks
predict a yellow supergiant duration an order of magnitude longer than we
observed. Here we turn our attention to the SMC, where the metallicity is 10x
lower than that of M31, which is important as metallicity strongly affects
massive star evolution. The SMC's large radial velocity (~160 km/s) allows us
to separate members from foreground stars. Observations of ~500 candidates
yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along
with 306 foreground stars and provide good relative numbers of yellow
supergiants down to 12Mo. Of the 176 near-certain SMC supergiants, the
kinematics predicted by the Besancon model of the Milky Way suggest a
foreground contamination of >4%. After placing the SMC supergiants on the H-R
diagram and comparing our results to the Geneva evolutionary tracks, we find
results similar to those of the M31 study: while the locations of the stars on
the H-R diagram match the locations of evolutionary tracks well, the models
over-predict the yellow supergiant lifetime by a factor of ten. Uncertainties
about the mass-loss rates on the main-sequence thus cannot be the primary
problem with the models.Comment: Accepted by the Ap
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
The Yellow and Red Supergiants of M33
Yellow and red supergiants are evolved massive stars whose numbers and
locations on the HR diagram can provide a stringent test for models of massive
star evolution. Previous studies have found large discrepancies between the
relative number of yellow supergiants observed as a function of mass and those
predicted by evolutionary models, while a disagreement between the predicted
and observed locations of red supergiants on the HR diagram was only recently
resolved. Here we extend these studies by examining the yellow and red
supergiant populations of M33. Unfortunately, identifying these stars is
difficult as this portion of the color-magnitude diagram is heavily
contaminated by foreground dwarfs. We identify the red supergiants through a
combination of radial velocities and a two-color surface gravity discriminant
and, after re-characterizing the rotation curve of M33 with our newly selected
red supergiants, we identify the yellow supergiants through a combination of
radial velocities and the strength of the OI 7774 triplet. We examine
~1300 spectra in total and identify 121 yellow supergiants (a sample which is
unbiased in luminosity above log(L/L\odot) ~ 4.8) and 189 red supergiants.
After placing these objects on the HR diagram, we find that the latest
generation of Geneva evolutionary tracks show excellent agreement with the
observed locations of our red and yellow supergiants, the observed relative
number of yellow supergiants with mass and the observed red supergiant upper
mass limit. These models therefore represent a drastic improvement over
previous generations.Comment: Accepted for publication in the Astrophysical Journa
The Discovery of a P Cygni Analog in M31
We present spectroscopy and discuss the photometric history of a previously
obscure star in M31. The spectrum of the star is an extremely close match to
that of P Cygni, one of the archetypes of Luminous Blue Variables (LBVs). The
star has not shown much variability over the past 40 years ( mag),
although small-scale (0.05 mag) variations over a year appear to be real.
Nevertheless, the presence of a sub-arcsecond extension around the star is
indicative of a past outburst, and from the nebula's size (0.5 pc diameter) we
estimate the outburst took place roughly 2000 yrs ago. P Cygni itself exhibits
a similar photometric behavior, and has a similar nebula (0.2 pc diameter). We
argue that this may be more typical behavior for LBVs than commonly assumed.
The star's location in the HR diagram offers substantial support for stellar
evolutionary models that include the effects of rotation, as the star is just
at a juncture in the evolutionary track of a 85Mo star. The star is likely in a
transition from an O star to a late-type WN Wolf-Rayet.Comment: To appear in ApJ (Letters
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