72 research outputs found
Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters II. M13 and M92
High resolution spectra of 123 red giant stars in the globular cluster M13
and 64 red giant stars in M92 were obtained with Hectochelle at the MMT
telescope. Emission and line asymmetries in Halpha, and Ca K are identified,
characterizing motions in the extended atmospheres and seeking differences
attributable to metallicity in these clusters and M15. On the red giant branch,
emission in Halpha generally appears in stars with T_eff < 4500 K and log
L/L_sun > 2.75. Fainter stars showing emission are asymptotic giant branch
(AGB) stars or perhaps binary stars. The line-bisector for Halpha reveals the
onset of chromospheric expansion in stars more luminous than log L/L_sun ~ 2.5
in all clusters, and this outflow velocity increases with stellar luminosity.
However, the coolest giants in the metal-rich M13 show greatly reduced outflow
in Halpha most probably due to decreased T_eff and changing atmospheric
structure. The Ca K_3 outflow velocities are larger than shown by Halpha at the
same luminosity and signal accelerating outflows in the chromospheres. Stars
clearly on the AGB show faster chromospheric outflows in Halpha than RGB
objects. While the Halpha velocities on the RGB are similar for all
metallicities, the AGB stars in the metal-poor M15 and M92 have higher outflow
velocities than in the metal-rich M13. Comparison of these chromospheric line
profiles in the paired metal-poor clusters, M15 and M92 shows remarkable
similarities in the presence of emission and dynamical signatures, and does not
reveal a source of the `second-parameter' effect.Comment: 41 pages, 14 figures, 11 tables, Accepted in Astronomical Journa
Probing 3D and NLTE models using APOGEE observations of globular cluster stars
Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effects
are known to affect abundance analyses. However, there are very few
observational abundance testsof 3D and NLTE models. We developed a new way of
testing the abundance predictions of 3D and NLTE models, taking advantage of
large spectroscopic survey data. We use a line-by-line analysis of the Apache
Point Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) with
the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS).
We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number of
globular cluster K giants in the APOGEE survey. We compare this line-by-line
analysis against NLTE and 3D predictions. While the 1D-NLTE models provide
corrections in the right direction, there are quantitative discrepancies
between different models. We observe a better agreement with the data for the
models including reliable collisional cross-sections. The agreement between
data and models is not always satisfactory when the 3D spectra are computed in
LTE. However, we note that for a fair comparison, 3D corrections should be
computed with self-consistently derived stellar parameters, and not on 1D
models with identical stellar parameters. Finally, we focus on 3D and NLTE
effects on Fe lines in the H band, where we observe a systematic difference in
abundance relative to the value from the optical. Our results suggest that the
metallicities obtained from the H band are more accurate in metal-poor giants.
More atomic data and extended self-consistent 3D-NLTE computations need to be
made. The method we have developed for testing 3D and NLTE models could be
extended to other lines and elements, and is particularly suited for large
spectroscopic surveys.Comment: 13 pages, 9 figures, accepted in A&
Mass Outflow from Red Giant Stars in M13, M15, and M92
Chromospheric model calculations of the Halpha line for selected red giant
branch (RGB) and asymptotic giant branch (AGB) stars in the globular clusters
M13, M15, and M92 are constructed to derive mass loss rates. The model spectra
are compared to the observations obtained with the Hectochelle on the MMT
telescope. These stars show strong Halpha emissions and blue-shifted Halpha
cores signaling that mass outflow is present in all stars. Outflow velocities
of 3-19 km/s, larger than indicated by Halpha profiles, are needed in the upper
chromosphere to achieve good agreement between the model spectra and the
observations. The resulting mass loss rates range from 0.6*10^{-9} to 5*10^{-9}
Msun/yr, which are about an order of magnitude lower than predicted from
"Reimers' law" or inferred from the infrared excess of similar stars. The mass
loss rate increases slightly with luminosity and with decreasing effective
temperature. Stars in the more metal-rich M13 have higher mass loss rates by a
factor of ~2 than in the metal-poor clusters M15 and M92. A fit to the mass
loss rates is given by: M [Msun/yr] = 0.092 * L^{0.16} * Teff^{-2.02} *
A^{0.37} where A=10^[Fe/H]. Multiple observations of stars revealed one object
in M15, K757, in which the mass outflow increased by a factor of 6 between two
observations separated by 18 months. Other stars showed changes in mass loss
rate by a factor of 1.5 or less.Comment: 28 pages, 10 figures, 3 tables, Accepted in Astronomical Journa
Ethnographic accounts of visitors from the Austro-Hungarian Monarchy to the Asian peripheries of Russia and their contribution to the development of systematic ethnological studies in the Monarchy: Preliminary results and research perspectives
The authors intend to provide an overview of the diaries, travelogues, and correspondence of Austro-Hungarians who traveled to the Asian peripheries of Russia during the Dual Monarchy. We aim to contribute to ongoing discussions on colonial discourses of otherness, as well as to the historical development of ethnographic scholarship in Europe. Travel writing, orientalism, and colonial encounters with Asian otherness are closely intermingling phenomena in the modern era. We argue that the rich corpus of visual and verbal representations of North-, Central-, and Inner-Asian peoples recorded by the subjects of the Dual Monarchy provides instructive examples of colonial encounters with non-colonizers in 19th century Asia. Furthermore, we believe that these examples will bring forth a more detailed picture of how the ideas born in the centers of German enlightenment (like Völkerkunde) impregnated the intellectual life of more peripheral regions in Europe. As ethnographic scholarship developed within national research traditions rather than in the frame of a monolithic, European intellectual project, our question is whether or not the Dual Monarchy provided a meaningful frame to bridge national research traditions
New ATLAS9 And MARCS Model Atmosphere Grids for the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
We present a new grid of model photospheres for the SDSS-III/APOGEE survey of
stellar populations of the Galaxy, calculated using the ATLAS9 and MARCS codes.
New opacity distribution functions were generated to calculate ATLAS9 model
photospheres. MARCS models were calculated based on opacity sampling
techniques. The metallicity ([M/H]) spans from -5 to 1.5 for ATLAS and -2.5 to
0.5 for MARCS models. There are three main differences with respect to previous
ATLAS9 model grids: a new corrected H2O linelist, a wide range of carbon
([C/M]) and alpha element [alpha/M] variations, and solar reference abundances
from Asplund et al. 2005. The added range of varying carbon and alpha element
abundances also extends the previously calculated MARCS model grids. Altogether
1980 chemical compositions were used for the ATLAS9 grid, and 175 for the MARCS
grid. Over 808 thousand ATLAS9 models were computed spanning temperatures from
3500K to 30000K and log g from 0 to 5, where larger temperatures only have high
gravities. The MARCS models span from 3500K to 5500K, and log g from 0 to 5.
All model atmospheres are publically available online.Comment: 8 pages, 6 figures, 5 tables, accepted for publication in The
Astronomical Journa
Exploring the circumstellar environment of the young eruptive star V2492 Cyg
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010.
The near-infrared color changes observed since the outburst peak suggest that
the source belongs to a newly defined sub-class of young eruptive stars, where
time-dependent accretion and variable line-of-sight extinction play a combined
role in the flux changes.
Aims. In order to learn about the origin of the light variations and to
explore the circumstellar and interstellar environment of V2492 Cyg, we
monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2
\mu m from the ground and between 3.6 \mu m and 160 \mu m from space.
Methods. We analyze the light curves and study the color-color diagrams via
comparison with the standard reddening path. We examine the structure of the
molecular cloud hosting V2492 Cyg by computing temperature and optical depth
maps from the far-infrared data.
Results. We find that the shapes of the light curves at different wavelengths
are strictly self-similar and that the observed variability is related to a
single physical process, most likely variable extinction. We suggest that the
central source is episodically occulted by a dense dust cloud in the inner
disk, and, based on the invariability of the far-infrared fluxes, we propose
that it is a long-lived rather than a transient structure. In some respects,
V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars.
Conclusions. The example of V2492 Cyg demonstrates that the light variations
of young eruptive stars are not exclusively related to changing accretion. The
variability provided information on an azimuthally asymmetric structural
element in the inner disk. Such an asymmetric density distribution in the
terrestrial zone may also have consequences for the initial conditions of
planet formation.Comment: 9 pages, 7 figures, 2 online tables, accepted for publication in A&
A photometric monitoring of bright high-amplitude delta Scuti stars. II. Period updates for seven stars
We present new photometric data for seven high-amplitude delta Scuti stars.
The observations were acquired between 1996 and 2002, mostly in the Johnson
photometric system. For one star (GW UMa), our observations are the first since
the discovery of its pulsational nature from the Hipparcos data.The primary
goal of this project was to update our knowledge on the period variations of
the target stars. For this, we have collected all available photometric
observations from the literature and constructed decades-long O-C diagrams of
the stars. This traditional method is useful because of the single-periodic
nature of the light variations. Text-book examples of slow period evolution (XX
Cyg, DY Her, DY Peg) and cyclic period changes due to light-time effect (LITE)
in a binary system (SZ Lyn) are updated with the new observations. For YZ Boo,
we find a period decrease instead of increase. The previously suggested
LITE-solution of BE Lyn (Kiss & Szatmary 1995) is not supported with the new
O-C diagram. Instead of that, we suspect the presence of transient light curve
shape variations mimicking small period changes.Comment: 11 pages, 15 figures, accepted for publication in A&
Cosmic variance in [O/Fe] in the Galactic disk
We examine the distribution of the [O/Fe] abundance ratio in stars across the Galactic disk using H-band spectra from the Apache Point Galactic Evolution Experiment (APOGEE). We minimize systematic errors by considering groups of stars with similar atmospheric parameters. The APOGEE measurements in the Sloan Digital Sky Survey data release 12 reveal that the square root of the star-to-star cosmic variance in the oxygen-to-iron ratio at a given metallicity is about 0.03–0.04 dex in both the thin and thick disk. This is about twice as high as the spread found for solar twins in the immediate solar neighborhood and the difference is probably associated to the wider range of galactocentric distances spanned by APOGEE stars. We quantify the uncertainties by examining the spread among stars with the same parameters in clusters; these errors are a function of effective temperature and metallicity, ranging between 0.005 dex at 4000 K and solar metallicity, to about 0.03 dex at 4500 K and [Fe/H] ≃ −0.6. We argue that measuring the spread in [O/Fe] and other abundance ratios provides strong constraints for models of Galactic chemical evolution
The first year of SN 2004dj in NGC 2403
New BVRI photometry and optical spectroscopy of the Type IIp supernova 2004dj
in NGC 2403, obtained during the first year since discovery, are presented. The
progenitor cluster, Sandage 96, is also detected on pre-explosion frames. The
light curve indicates that the explosion occured about 30 days before
discovery, and the plateau phase lasted about +110 \pm 20 days after that. The
plateau-phase spectra have been modelled with the SYNOW spectral synthesis code
using H, NaI, TiII, ScII, FeII and BaII lines. The SN distance is inferred from
the Expanding Photosphere Method and the Standard Candle Method applicable for
SNe IIp. They resulted in distances that are consistent with each other as well
as earlier Cepheid- and Tully-Fisher distances. The average distance, D = 3.47
\pm 0.29 Mpc is proposed for SN 2004dj and NGC 2403. The nickel mass produced
by the explosion is estimated as 0.02 \pm 0.01 M_o. The SED of the progenitor
cluster is reanalysed by fitting population synthesis models to our observed
BVRI data supplemented by U and JKH magnitudes from the literature. The
chi^2-minimization revealed a possible "young" solution with cluster age T_{cl}
= 8 Myr, and an "old" solution with T_{cl} = 20 - 30 Myr. The "young" solution
would imply a progenitor mass M > 20 M_o, which is higher than the previously
detected progenitor masses for Type II SNe.Comment: 19 pages, accepted in MNRA
- …