389 research outputs found
Eighth Grade: Found Footage and Found Story
An in-depth examination of Eighth Grade\u27s (Burhnam, 2017) relationships with the concept of Kracauer\u27s Found Story and the Found Footage subgenre
A K-band spectral mini-survey of Galactic B[e] stars
We present a mini-survey of Galactic B[e] stars mainly undertaken with the
Large Binocular Telescope (LBT). B[e] stars show morphological features with
hydrogen emission lines and an infrared excess, attributed to warm
circumstellar dust. In general, these features are assumed to arise from dense,
non-spherical, disk-forming circumstellar material in which molecules and dust
can condensate. Due to the lack of reliable luminosities, the class of Galactic
B[e] stars contains stars at very different stellar evolutionary phases like
Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit
K-band spectra for a sample of Galactic B[e] stars with the LBT-Luci I.
Prominent spectral features, such as the Brackett gamma line and CO band heads
are identified in the spectra. The analysis shows that the stars can be
characterized as evolved objects. Among others we find one LBV candidate
(MWC314), one supergiant B[e] candidate with 13CO (MWC137) and in two cases
(MWC623 and AS 381) indications for the existence of a late-type binary
companion, complementary to previous studies. For MWC84, IR spectra were taken
at different epochs with LBT-Luci I and the GNIRS spectrograph at the Gemini
North telescope. The new data show the disappearance of the circumstellar CO
emission around this star, previously detectable over decades. Also no signs of
a recent prominent eruption leading to the formation of new CO disk emission
are found during 2010 and 2013.Comment: 10 pages, 7 figures, 4 tables, accepted for publication in MNRAS (in
press
The Quintuplet Cluster III. Hertzsprung-Russell diagram and cluster age
The Quintuplet, one of three massive stellar clusters in the Galactic center,
is located about 30pc in projection from Sagittarius A*. Based on near-infrared
K-band spectra we determine temperatures and luminosities for all stars in our
sample and construct the Herztsprung-Russell diagram. We find two distinct
groups: early-type OB stars and late-type KM stars, well separated from each
other. By comparison with Geneva stellar evolution models we derive initial
masses exceeding 8 solar masses for the OB stars, that are located along an
isochrone corresponding to a cluster age of about 4 million years. In addition,
we derive number ratios (e. g. N_WR/N_O) and compare them with predictions of
population synthesis models. We find that an instantaneous burst of star
formation at about 3.3 to 3.6\,Myr ago is the most likely scenario to form the
Quintuplet cluster. The late-type stars in the sample are red giant branch
(RGB) stars or red supergiants (RSGs) according to their spectral signatures.
It is discussed if they could physically belong to the Quintuplet cluster.
Furthermore, we apply a mass-luminosity relation to construct the initial mass
function (IMF) of the cluster. We find indications for a slightly top-heavy
IMF.Comment: 10 pages, 9 figures, 2 tables, accepted for publication in A&
The Wolf-Rayet stars in the Large Magellanic Cloud: A comprehensive analysis of the WN class
Aims: Following our comprehensive studies of the WR stars in the Milky Way,
we now present spectroscopic analyses of almost all known WN stars in the LMC.
Methods: For the quantitative analysis of the wind-dominated emission-line
spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By
fitting synthetic spectra to the observed spectral energy distribution and the
available spectra (ultraviolet and optical), we obtain the physical properties
of 107 stars. Results: We present the fundamental stellar and wind parameters
for an almost complete sample of WN stars in the LMC. Among those stars that
are putatively single, two different groups can be clearly distinguished. While
12% of our sample are more luminous than 10^6 Lsun and contain a significant
amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate
the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the
few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars,
descended directly from the main sequence at very high initial masses, the bulk
of WN stars have gone through the red-supergiant phase. According to their
luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate
from initial masses between 20 and 40 Msun. This mass range is similar to the
one found in the Galaxy, i.e. the expected metallicity dependence of the
evolution is not seen. Current stellar evolution tracks, even when accounting
for rotationally induced mixing, still partly fail to reproduce the observed
ranges of luminosities and initial masses. Moreover, stellar radii are
generally larger and effective temperatures correspondingly lower than
predicted from stellar evolution models, probably due to subphotospheric
inflation.Comment: 17+46 pages; 10+54 figures; v2: typos corrected, space-saving layout
for appendix C, published in A&
A K-band spectral mini-survey of Galactic B[e] stars
We present a mini-survey of Galactic B[e] stars mainly undertaken with the Large Binocular Telescope (LBT). B[e] stars show morphological features with hydrogen emission lines and an infrared excess, attributed to warm circumstellar dust. In general, these features are assumed to arise from dense, non-spherical, disc-forming circumstellar material in which molecules and dust can condensate. Due to the lack of reliable luminosities, the class of Galactic B[e] stars contains stars at very different stellar evolutionary phases like Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit K-band spectra for a sample of Galactic B[e] stars with the LBT-LUCI 1. Prominent spectral features, such as the Brackett Îł line and CO band heads are identified in the spectra. The analysis shows that the stars can be characterized as evolved objects. Among others we find one luminous blue variable candidate (MWC314), one supergiant B[e]candidate with 13CO (MWC 137), and in two cases (MWC 623 and AS 381) indications for the existence of a late-type binary companion, complementary to previous studies. For MWC 84, IR spectra were taken at different epochs with LBT-LUCI 1 and the GNIRS spectrograph at the Gemini North telescope. The new data show the disappearance of the circumstellar CO emission around this star, previously detectable over decades. Also no signs of a recent prominent eruption leading to the formation of new CO disc emission are found during 2010 and 2013.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto de AstrofĂsica de La Plat
High-pressure behavior and P-induced phase transition of CaB3O4(OH)3·H2O (colemanite)
Colemanite (ideally CaB3O4(OH)3\ub7H2O, space group P21/a, unit-cell parameters: a ~ 8.74, b ~ 11.26, c ~ 6.10 \uc5, \u3b2 ~ 110.1\ub0) is one of the principal mineralogical components of borate deposits and the most important mineral commodity of boron. Its high-pressure behavior is here described, for the first time, by means of in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell up to 24 GPa (and 293 K). Colemanite is stable, in its ambient-conditions polymorph, up to 13.95 GPa. Between 13.95 and 14.91 GPa, an iso-symmetric first-order single-crystal to single-crystal phase transition (reconstructive in character) toward a denser polymorph (colemanite-II) occurs, with: aCOL-II=3\ub7aCOL, bCOL-II=bCOL, and cCOL-II=2\ub7cCOL. Up to 13.95 GPa, the bulk compression of colemanite is accommodated by the Ca-polyhedron compression and the tilting of the rigid three-membered rings of boron polyhedra. The phase transition leads to an increase in the average coordination number of both the B and Ca sites. A detailed description of the crystal structure of the high-P polymorph, compared to the ambient-conditions colemanite, is given. The elastic behaviors of colemanite and of its high-P polymorph are described by means of III- and II-order Birch-Murnaghan equations of state, respectively, yielding the following refined parameters: KV0=67(4) GPa and KV\u2032=5.5(7) [\u3b2V0=0.0149(9) GPa-1] for colemanite; KV0=50(8) GPa [\u3b2V0=0.020(3) GPa-1] for its high-P polymorph
Universal phase transitions of B1 structured stoichiometric transition-metal carbides
The high-pressure phase transitions of B1-structured stoichiometric
transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were
systematically investigated using ab initio calculations. These carbides
underwent universal phase transitions along two novel phase-transition routes,
namely, B1\rightarrowdistorted TlI (TlI')\rightarrowTlI and/or
B1\rightarrowdistorted TiB (TiB')\rightarrowTiB, when subjected to pressures.
The two routes can coexist possibly because of the tiny enthalpy differences
between the new phases under corresponding pressures. Four new phases result
from atomic slips of the B1-structured parent phases under pressure. After
completely releasing the pressure, taking TiC as a representative of TMCs, only
its new TlI'-type phase is mechanically and dynamically stable, and may be
recovered.Comment: [email protected]
The Carbon footprint of B[e] supergiants
We report on the first detection of C enhancement in two B[e]
supergiants in the Large Magellanic Cloud. Stellar evolution models predict the
surface abundance in C to strongly increase during main-sequence and
post-main sequence evolution of massive stars. However, direct identification
of chemically processed material on the surface of B[e] supergiants is hampered
by their dense, disk-forming winds, hiding the stars. Recent theoretical
computations predict the detectability of enhanced C via the molecular
emission in CO arising in the circumstellar disks of B[e] supergiants.
To test this potential method and to unambiguously identify a post-main
sequence B[e]SG by its CO emission, we have obtained high-quality
-band spectra of two known B[e] supergiants in the Large Magellanic Cloud,
using the Very Large Telescope's Spectrograph for INtegral Field Observation in
the Near-Infrared (VLT/SINFONI). Both stars clearly show the CO band
emission, whose strength implies a strong enhancement of C, in agreement
with theoretical predictions. This first ever direct confirmation of the
evolved nature of B[e] supergiants thus paves the way to the first
identification of a Galactic B[e] supergiant.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
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