653 research outputs found
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
Reassessment of the TM 1517 odonto-postcranial assemblage from Kromdraai B, South Africa, and the maturational pattern of Paranthropus robustus
Objectives
The Pleistocene taxon Paranthropus robustus was established in 1938 following the discovery at Kromdraai B, South Africa, of the partial cranium TM 1517a and associated mandible TM 1517b. Shortly thereafter, a distal humerus (TM 1517g), a proximal ulna (TM 1517e), and a distal hallucial phalanx (TM 1517k) were collected nearby at the site, and were considered to be associated with the holotype. TM 1517aâb represents an immature individual; however, no analysis of the potentially associated postcranial elements has investigated the presence of any endostructural remnant of recent epiphyseal closure. This study aims at tentatively detecting such traces in the three postcranial specimens from Kromdraai B.
Materials and Methods
By using ÎŒXCT techniques, we assessed the developmental stage of the TM 1517b's CâM3 roots and investigated the inner structure of TM 1517g, TM 1517e, and TM 1517k.
Results
The M2 shows incompletely closed root apices and the M3 a halfâcompleted root formation stage. The distal humerus was likely completely fused, while the proximal ulna and the distal hallucial phalanx preserve endosteal traces of the diaphyseoâepiphyseal fusion process.
Discussion
In the hominin fossil record, there are few unambiguously associated craniodental and postcranial remains sampling immature individuals, an essential condition for assessing the taxonâspecific maturational patterns. Our findings corroborate the original association of the craniodental and postcranial remains representing the P. robustus type specimen. As with other PlioâPleistocene hominins, the odontoâpostcranial maturational pattern of TM 1517 more closely fits an African great ape rather than the extant human pattern
An analysis of ultraviolet spectra of Extreme Helium Stars and new clues to their origins
Abundances of about 18 elements including the heavy elements Y and Zr are
determined from Hubble Space Telescope Space Telescope Imaging Spectrograph
ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179,
V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of
the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The
abundance analyses is done using LTE line formation and LTE model atmospheres
especially constructed for these EHe stars. The stellar parameters derived from
an EHe's UV spectrum are in satisfactory agreement with those derived from its
optical spectrum. Adopted abundances for the seven EHes are from a combination
of the UV and optical analyses. Published results for an additional ten EHes
provide abundances obtained in a nearly uniform manner for a total of 17 EHes,
the largest sample on record. The initial metallicity of an EHe is indicated by
the abundance of elements from Al to Ni; Fe is adopted to be the representative
of initial metallicity. Iron abundances range from approximately solar to about
one-hundredth of solar. Clues to EHe evolution are contained within the H, He,
C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for
some stars is close to the predicted initial abundance yet the N abundance
indicates almost complete conversion of initial C, N, and O to N by the
CNO-cycles; (ii) three of the seven stars with UV spectra show a strong
enhancement of Y and Zr attributable to an s-process. The observed compositions
are discussed in light of expectations from accretion of a He white dwarf by a
CO white dwarf.Comment: 126 pages, 15 figures, 20 Tables, accepted for publication in the Ap
Opening the Treasure Chest: A Newborn Star Cluster Emerges from its Dust Pillar in Carina
We present detailed observations of the Treasure Chest, a compact nebula at
the head of a dust pillar in the southern Carina nebula. This object is of
interest because it is an example of a dense young cluster containing at least
one massive star, the formation of which may have been triggered by feedback
from the very massive stars in the Carina nebula, and possibly Eta Car itself.
We investigate both the nebular and stellar content of the object. Near-IR
emission-line images reveal a cavity inside the head of the dust pillar, which
contains a dense cluster of young stars, while the observed spectrum of the
nebula is consistent with an H II region ionized by an O9.5 V star. After
subtracting contamination of field stars within the Carina nebula itself, we
compare the cluster's color magnitude diagram with pre-main-sequence isochrones
to derive a likely cluster age less than about 0.1 Myr. This is in reasonable
agreement with the dynamical age of a few times 10^4 yr for the expanding
nebular cavity, indicating extreme youth. Stars in the Treasure Chest cluster
are highly reddened, with extinction values as large as A_V=50. Two-thirds of
cluster members show strong infrared excess colors indicative of circumstellar
disks, which may prove to be among the highest fraction yet seen for a young
cluster. All evidence suggests that the Treaure Chest is an extremely young
cluster that is just now breaking out of its natal cloud into the surrounding
massive star forming region, and is a good target for more detailed study.Comment: 23 pages, 10 figs, 4 color fig
The Distances to Open Clusters from Main-Sequence Fitting. IV. Galactic Cepheids, the LMC, and the Local Distance Scale
We derive the basic properties of seven Galactic open clusters containing
Cepheids and construct their period-luminosity (P-L) relations. For our cluster
main-sequence fitting we extend previous Hyades-based empirical
color-temperature corrections to hotter stars using the Pleiades as a template.
We use BVI_{C}JHK_{s} data to test the reddening law, and include metallicity
effects to perform a more comprehensive study for our clusters than prior
efforts. The ratio of total to selective extinction R_V that we derive is
consistent with expectations. Assuming the LMC P-L slopes, we find =
-3.93 +/- 0.07 (statistical) +/- 0.14 (systematic) for 10-day period Cepheids,
which is generally fainter than those in previous studies. Our results are
consistent with recent HST and Hipparcos parallax studies when using the
Wesenheit magnitudes W(VI). Uncertainties in reddening and metallicity are the
major remaining sources of error in the V-band P-L relation, but a higher
precision could be obtained with deeper optical and near-infrared cluster
photometry. We derive distances to NGC4258, the LMC, and M33 of (m - M)_0 =
29.28 +/- 0.10, 18.34 +/- 0.06, and 24.55 +/- 0.28, respectively, with an
additional systematic error of 0.16 mag in the P-L relations. The distance to
NGC4258 is in good agreement with the geometric distance derived from water
masers [\Delta (m - M)_0 = 0.01 +/- 0.24]; our value for M33 is less consistent
with the distance from an eclipsing binary [\Delta (m - M)_0 = 0.37 +/- 0.34];
our LMC distance is moderately shorter than the adopted distance in the HST Key
Project, which formally implies an increase in the Hubble constant of 7% +/-
8%.Comment: 28 pages, 21 figures; accepted for publication in the Ap
MOST detects SPBe pulsations in HD 127756 & HD 217543: Asteroseismic rotation rates independent of vsini
The MOST (Microvariability and Oscillations of Stars) satellite has
discovered SPBe (Slowly Pulsating Be) oscillations in the stars HD 127756
(B1/B2 Vne) and HD 217543 (B3 Vpe). For HD 127756, 30 significant frequencies
are identified from 31 days of nearly continuous photometry; for HD 217543, up
to 40 significant frequencies from 26 days of data. In both cases, the
oscillations fall into three distinct frequency ranges, consistent with models
of the stars. The variations are caused by nonradial g-modes (and possibly
r-modes) distorted by rapid rotation and excited by the opacity mechanism near
the iron opacity bump. A comparison of pulsation models and observed frequency
groups yields a rotation frequency for each star, independently of vsini. The
rotation rates of these stars, as well as those of the SPBe stars previously
discovered by MOST, HD 163868 and CMi, are all close to their critical
values.Comment: 36 pages, 16 figures, 3 tables, accepted for publication in Ap
Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
We report near-IR spectroscopic observations of the Eta Carinae massive
binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad
absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0
spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a
similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines
of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization
species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to
-1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s
originates in a region markedly faster and more ionized than the nominal wind
of the primary star. Observations obtained at the OPD/LNA during the last 4
spectroscopic cycles (1989-2009) also display high-velocity absorption in He I
10833 during periastron. Based on the OPD/LNA dataset, we determine that
material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023
of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi
> 1.049. Therefore, we constrain the duration of the high-velocity absorption
to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the
high-velocity absorption originates from shocked gas in the wind-wind collision
zone, at distances of 15 to 45 AU in the line-of-sight to the primary star.
Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find
that the dense high-velocity gas is in the line-of-sight to the primary star
only if the binary system is oriented in the sky so that the companion is
behind the primary star during periastron, corresponding to a longitude of
periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the
orbital plane relative to the Homunculus equatorial plane and conclude that our
data are broadly consistent with orbital inclinations in the range i=40 to 60
degrees.Comment: 18 pages, 15 figures, accepted for publication in A&A;
high-resolution PDF version available also at
http://www.mpifr.de/staff/jgroh/etacar.htm
Lithium storage mechanisms in purpurin based organic lithium ion battery electrodes
Current lithium batteries operate on inorganic insertion compounds to power a diverse range of
applications, but recently there is a surging demand to develop environmentally friendly green electrode
materials. To develop sustainable and eco-friendly lithium ion batteries, we report reversible lithium ion
storage properties of a naturally occurring and abundant organic compound purpurin, which is non-toxic
and derived from the plant madder. The carbonyl/hydroxyl groups present in purpurin molecules act as
redox centers and reacts electrochemically with Li-ions during the charge/discharge process. The
mechanism of lithiation of purpurin is fully elucidated using NMR, UV and FTIR spectral studies. The
formation of the most favored six membered binding core of lithium ion with carbonyl groups of purpurin
and hydroxyl groups at C-1 and C-4 positions respectively facilitated lithiation process, whereas hydroxyl
group at C-2 position remains unaltered
Unravelling structural ambiguities in lithium- and manganese-rich transition metal oxides
Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects
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