421 research outputs found
Beryllium and Alpha-Element Abundances in a Large Sample of Metal-Poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects of
astronomy including stellar structure, Galactic evolution, and cosmology. We
have taken spectra of Be in 117 metal-poor stars ranging in metallicity from
[Fe/H] = -0.5 to -3.5 with Keck I + HIRES at a resolution of 42,000 and
signal-to-noise ratios of near 100. We have determined the stellar parameters
spectroscopically from lines of Fe I, Fe II, Ti I and Ti II. The abundances of
Be and O were derived by spectrum synthesis techniques, while abundances of Fe,
Ti, and Mg were found from many spectral line measurements. There is a linear
relationship between [Fe/H] and A(Be) with a slope of +0.88 +-0.03 over three
orders of magnitude in [Fe/H]. We fit the relationship between A(Be) and [O/H]
with both a single slope and with two slopes. The relationship between [Fe/H]
and [O/H] seems robustly linear and we conclude that the slope change in Be vs.
O is due to the Be abundance. Although Be is a by-product of CNO, we have used
Ti and Mg abundances as alpha-element surrogates for O in part because O
abundances are rather sensitive to both stellar temperature and surface
gravity. We find that A(Be) tracks [Ti/H] very well with a slope of 1.00
+-0.04. It also tracks [Mg/H] very well with a slope of 0.88 +-0.03. We find
that there are distinct differences in the relationships of A(Be) and [Fe/H]
and of A(Be) and [O/H] for our dissipative stars and our accretive stars. We
suggest that the Be in the dissipative stars was primarily formed by GCR
spallation and Be in the accretive stars was formed in the vicinity of SN II.Comment: Accepted for Ap.J. Nov. 10, 2011, v. 741 70 pages, 27 figures, 5
table
Nitrogen Production in Starburst Galaxies Detected by GALEX
We investigate the production of nitrogen in star-forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 8745 GALEX emission-line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. We derive both gas-phase oxygen and nitrogen abundances for the sample and apply stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. We compare oxygen abundances derived using three different diagnostics. We derive the specific star formation rates of the galaxies by modeling the seven-band GALEX+SDSS photometry. We find that galaxies that have log (SFR/M_*) ≳ − 10.0 typically have values of log (N/O) ~ 0.05 dex less than galaxies with log (SFR/M_*) ≾ − 10.0 and similar oxygen abundances
UV Star Formation Rates in the Local Universe
We measure star formation rates of ~50,000 optically-selected galaxies in the
local universe (z~0.1), spanning a range from gas-rich dwarfs to massive
ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS
(optical) photometry to a library of population synthesis models that include
dust attenuation. For star-forming galaxies, our UV-based SFRs compare
remarkably well with those derived from SDSS H alpha. Deviations from perfect
agreement between these two methods are due to differences in the dust
attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for
galaxies with weak or no H alpha emission, and where H alpha is contaminated
with an emission from an AGN. We use full-SED SFRs to calibrate a simple
prescription that uses GALEX UV magnitudes to produce good SFRs for normal
star-forming galaxies. The specific SFR is considered as a function of stellar
mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and
for (3) galaxies without H alpha emission. We find that the three have distinct
star formation histories, with AGN lying intermediate between the star-forming
and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie
on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong
AGN appear to represent the massive continuation of this sequence. Weak AGN,
while also massive, have lower SFR, sometimes extending to the realm of
quiescent galaxies. We propose an evolutionary sequence for massive galaxies
that smoothly connects normal star-forming galaxies to quiescent (red sequence)
galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha
emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at
z=0.1 with smaller total error than previous measurements (abridged).Comment: Accepted for publication in ApJ (Special GALEX Supplement issue - Dec
2007). v2: Typo in Eq. 2 correcte
The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor
Luminous Red Novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system's common envelope (CE) shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of M_r = −10.97 ± 0.11 and maintained this brightness during its optical plateau of t_p = 41 ± 5days. During this phase, it showed a rather stable photospheric temperature of ~3300K and a luminosity of ~10⁴⁰ erg s⁻¹. The photosphere of AT2018bwo at early times appeared larger and cooler than other similar LRNe, likely due to an extended mass-loss episode before the merger. Towards the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The reprocessed emission by the cooling dust was also detected in the mid-infrared bands ~1.5 years after the outburst. Archival Spitzer and Hubble Space Telescope data taken 10-14 years before the transient event suggest a progenitor star with T_(prog) ∼ 6500K, R_(prog) ∼ 100 R_⊙ and L_(prog) ∼ 2 × 10⁴ L_⊙, and an upper limit for optically thin warm (1000 K) dust mass of M_d < 10⁻⁶ M_⊙. Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT2018bwo, we infer a primary mass of 12-16 M_⊙, which is 9-45% larger than the ~11M⊙ obtained using single-star evolution models. The system, consistent with a yellow-supergiant primary, was likely in a stable mass-transfer regime with -2.4 < log (Ṁ/M_⊙ yr⁻¹) < -1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15-0.5M⊙ with a velocity of ~500 km s⁻¹
A retrospective analysis of survival and prognostic factors after stereotactic radiosurgery for aggressive meningiomas
BACKGROUND: While most meningiomas are benign, aggressive meningiomas are associated with high levels of recurrence and mortality. A single institution’s Gamma Knife radiosurgical experience with atypical and malignant meningiomas is presented, stratified by the most recent WHO classification. METHODS: Thirty-one patients with atypical and 4 patients with malignant meningiomas treated with Gamma Knife radiosurgery between July 2000 and July 2011 were retrospectively reviewed. All patients underwent prior surgical resection. Overall survival was the primary endpoint and rate of disease recurrence in the brain was a secondary endpoint. Patients who had previous radiotherapy or prior surgical resection were included. Kaplan-Meier and Cox proportional hazards models were used to estimate survival and identify factors predictive of recurrence and survival. RESULTS: Post-Gamma Knife recurrence was identified in 11 patients (31.4%) with a median overall survival of 36 months and progression-free survival of 25.8 months. Nine patients (25.7%) had died. Three-year overall survival (OS) and progression-free survival (PFS) rates were 78.0% and 65.0%, respectively. WHO grade II 3-year OS and PFS were 83.4% and 70.1%, while WHO grade III 3-year OS and PFS were 33.3% and 0%. Recurrence rate was significantly higher in patients with a prior history of benign meningioma, nuclear atypia, high mitotic rate, spontaneous necrosis, and WHO grade III diagnosis on univariate analysis; only WHO grade III diagnosis was significant on multivariate analysis. Overall survival was adversely affected in patients with WHO grade III diagnosis, prior history of benign meningioma, prior fractionated radiotherapy, larger tumor volume, and higher isocenter number on univariate analysis; WHO grade III diagnosis and larger treated tumor volume were significant on multivariate analysis. CONCLUSION: Atypical and anaplastic meningiomas remain difficult tumors to treat. WHO grade III diagnosis and treated tumor volume were significantly predictive of recurrence and survival on multivariate analysis in aggressive meningioma patients treated with radiosurgery. Larger tumor size predicts poor survival, while nuclear atypia, necrosis, and increased mitotic rate are risk factors for recurrence. Clinical and pathologic predictors may help identify patients that are at higher risk for recurrence
Publisher Correction: Deep coverage whole genome sequences and plasma lipoprotein(a) in individuals of European and African ancestries.
The original version of this article contained an error in the name of the author Ramachandran S. Vasan, which was incorrectly given as Vasan S. Ramachandran. This has now been corrected in both the PDF and HTML versions of the article
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Deep coverage whole genome sequences and plasma lipoprotein(a) in individuals of European and African ancestries.
Lipoprotein(a), Lp(a), is a modified low-density lipoprotein particle that contains apolipoprotein(a), encoded by LPA, and is a highly heritable, causal risk factor for cardiovascular diseases that varies in concentrations across ancestries. Here, we use deep-coverage whole genome sequencing in 8392 individuals of European and African ancestry to discover and interpret both single-nucleotide variants and copy number (CN) variation associated with Lp(a). We observe that genetic determinants between Europeans and Africans have several unique determinants. The common variant rs12740374 associated with Lp(a) cholesterol is an eQTL for SORT1 and independent of LDL cholesterol. Observed associations of aggregates of rare non-coding variants are largely explained by LPA structural variation, namely the LPA kringle IV 2 (KIV2)-CN. Finally, we find that LPA risk genotypes confer greater relative risk for incident atherosclerotic cardiovascular diseases compared to directly measured Lp(a), and are significantly associated with measures of subclinical atherosclerosis in African Americans
Improved Standardization of Type II-P Supernovae: Application to an Expanded Sample
In the epoch of precise and accurate cosmology, cross-confirmation using a
variety of cosmographic methods is paramount to circumvent systematic
uncertainties. Owing to progenitor histories and explosion physics differing
from those of Type Ia SNe (SNe Ia), Type II-plateau supernovae (SNe II-P) are
unlikely to be affected by evolution in the same way. Based on a new analysis
of 17 SNe II-P, and on an improved methodology, we find that SNe II-P are good
standardizable candles, almost comparable to SNe Ia. We derive a tight Hubble
diagram with a dispersion of 10% in distance, using the simple correlation
between luminosity and photospheric velocity introduced by Hamuy & Pinto 2002.
We show that the descendent method of Nugent et al. 2006 can be further
simplified and that the correction for dust extinction has low statistical
impact. We find that our SN sample favors, on average, a very steep dust law
with total to selective extinction R_V<2. Such an extinction law has been
recently inferred for many SNe Ia. Our results indicate that a distance
measurement can be obtained with a single spectrum of a SN II-P during the
plateau phase combined with sparse photometric measurements.Comment: ApJ accepted version. Minor change
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