49 research outputs found
The long-term spectroscopic misadventures of AG Dra with a nod toward V407 Cyg: Degenerates behaving badly
We present some results of an ongoing study of the long-term spectroscopic
variations of AG Dra, a prototypical eruptive symbiotic system. We discuss the
effects of the environment and orbital modulation in this system and some of
the physical processes revealed by a comparison with the nova outburst of the
symbiotic-like recurrent nova V407 Cyg 2010.Comment: 11 pages, invited review to appear in the proceedings of the 2011
Asiago Workshop on Symbiotic Stars, A. Siviero and U. Munari eds., Baltic
Astronomy special issue. See also
http://etd.adm.unipi.it/theses/available/etd-05052010-002805
Classical Cepheids, what else?
We present new and independent estimates of the distances to the Magellanic
Clouds (MCs) using near-infrared (NIR) and optical--NIR period--Wesenheit (PW)
relations. The slopes of the PW relations are, within the dispersion, linear
over the entire period range and independent of metal content. The absolute
zero points were fixed using Galactic Cepheids with distances based on the
infrared surface-brightness method. The true distance modulus we found for the
Large Magellanic Cloud--- mag---and the
Small Magellanic Cloud--- mag---agree quite
well with similar distance determinations based on robust distance indicators.
We also briefly discuss the evolutionary and pulsation properties of MC
Cepheids
The Long-Term Spectroscopic Misadventures of AG Dra with a Nod toward V407 Cyg: Degenerates Behaving Badly
We present some results of an ongoing study of the long-term spectroscopic variations of AG Dra, a prototypical eruptive symbiotic system. We discuss the effects of the environment and orbital modulation in this system and some of the physical processes revealed by a comparison with the nova outburst of the symbiotic-like recurrent nova V407 Cyg 2010
The spectroscopic evolution of the symbiotic star AG Draconis. I.The O VI Raman, Balmer, and helium emission line variations during the outburst of 2006-2008
AG Dra is one of a small group of low metallicity S-type symbiotic binaries
with K-type giants that undergoes occasional short-term outbursts of unknown
origin. Our aim is to study the behavior of the white dwarf during an outburst
using the optical Raman lines and other emission features in the red giant
wind. The goal is to determine changes in the envelope and the wind of the
gainer in this system during a major outburst event and to study the coupling
between the UV and optical during a major outburst. Using medium and high
resolution groundbased optical spectra and comparisons with archival and
spectra, we study the evolution of the Raman O VI features and the
Balmer, He I, and He II lines during the outburst from 2006 Sept. through 2007
May and include more recent observations (2009) to study the subsequent
evolution of the source. The O VI Raman features disappeared completely at the
peak of the major outburst and the subsequent variation differs substantially
from that reported during the previous decade. The He I and He II lines, and
the Balmer lines, vary in phase with the Raman features but there is a
double-valuedness to the He I 6678, 7065 relative to the O VI Raman 6825\AA\
variations in the period between 2006-2008 that has not been previously
reported. The variations in the Raman feature ratio through the outburst
interval are consistent with the disappearance of the O VI FUV resonance wind
lines from the white dwarf and of the surrounding O ionized region
within the red giant wind provoked by the expansion and cooling of the white
dwarf photosphere.Comment: 10 pages, 15 figs. A&A (in press, accepted for publication
23/11/2009
On the distance of the Magellanic Clouds using Cepheid NIR and optical-NIR Period Wesenheit Relations
We present the largest near-infrared (NIR) data sets, , ever collected
for classical Cepheids in the Magellanic Clouds (MCs). We selected fundamental
(FU) and first overtone (FO) pulsators, and found 4150 (2571 FU, 1579 FO)
Cepheids for Small Magellanic Cloud (SMC) and 3042 (1840 FU, 1202 FO) for Large
Magellanic Cloud (LMC). Current sample is 2--3 times larger than any sample
used in previous investigations with NIR photometry. We also discuss optical
photometry from OGLE-III. NIR and optical--NIR Period-Wesenheit (PW)
relations are linear over the entire period range () and their slopes are, within the intrinsic dispersions, common between the
MCs. These are consistent with recent results from pulsation models and
observations suggesting that the PW relations are minimally affected by the
metal content. The new FU and FO PW relations were calibrated using a sample of
Galactic Cepheids with distances based on trigonometric parallaxes and Cepheid
pulsation models. By using FU Cepheids we found a true distance moduli of
mag (LMC) and
mag (SMC). These estimates
are the weighted mean over ten PW relations and the systematic errors account
for uncertainties in the zero-point and in the reddening law. We found similar
distances using FO Cepheids
( mag [LMC] and
mag [SMC]). These new MC
distances lead to the relative distance, mag (FU, ) and mag (FO, ),which agrees quite
well with previous estimates based on robust distance indicators.Comment: 17 pages, 7 figure
On the metallicity distribution of classical Cepheids in the Galactic inner disk
We present homogeneous and accurate iron abundances for almost four dozen
(47) of Galactic Cepheids using high-spectral resolution (R40,000) high
signal-to-noise ratio (S/N 100) optical spectra collected with UVES at
VLT. A significant fraction of the sample (32) is located in the inner disk (RG
6.9 kpc) and for half of them we provide new iron abundances. Current
findings indicate a steady increase in iron abundance when approaching the
innermost regions of the thin disk. The metallicity is super-solar and ranges
from 0.2 dex for RG 6.5 kpc to 0.4 dex for RG 5.5 kpc. Moreover,
we do not find evidence of correlation between iron abundance and distance from
the Galactic plane. We collected similar data available in the literature and
ended up with a sample of 420 Cepheids. Current data suggest that the mean
metallicity and the metallicity dispersion in the four quadrants of the
Galactic disk attain similar values. The first-second quadrants show a more
extended metal-poor tail, while the third-fourth quadrants show a more extended
metal-rich tail, but the bulk of the sample is at solar iron abundance.
Finally, we found a significant difference between the iron abundance of
Cepheids located close to the edge of the inner disk ([Fe/H]0.4) and
young stars located either along the Galactic bar or in the nuclear bulge
([Fe/H]0). Thus suggesting that the above regions have had different
chemical enrichment histories. The same outcome applies to the metallicity
gradient of the Galactic bulge, since mounting empirical evidence indicates
that the mean metallicity increases when moving from the outer to the inner
bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids
based on high-spectral resolution (R~38,000) high signal-to-noise ratio
(SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above
abundances were complemented with high-quality iron abundances provided either
by our group (86) or available in the literature. We paid attention in deriving
a common metallicity scale and ended up with a sample of 450 Cepheids. We also
estimated for the entire sample accurate individual distances by using
homogeneous near-infrared photometry and the reddening free Period-Wesenheit
relations. The new metallicity gradient is linear over a broad range of
Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar
estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover
evidence which suggests that the residuals of the metallicity gradient are
tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have
been identified as spatial overdensities of Cepheids located across the thin
disk. They account for a significant fraction of the residual fluctuations, and
in turn for the large intrinsic dispersion of the metallicity gradient. We
performed a detailed comparison with metallicity gradients based on different
tracers: OB stars and open clusters. We found very similar metallicity
gradients for ages younger than 3 Gyrs, while for older ages we found a
shallower slope and an increase in the intrinsic spread. The above findings
rely on homogeneous age, metallicity and distance scales. Finally we found, by
using a large sample of Galactic and Magellanic Cepheids for which are
available accurate iron abundances, that the dependence of the luminosity
amplitude on metallicity is vanishing.Comment: Accepted in A&A, 11 figures, 7 table
On the alpha-element gradients of the Galactic thin disk using Cepheids
We present new homogeneous measurements of Na, Al and three alpha-elements
(Mg, Si, Ca) for 75 Galactic Cepheids. The abundances are based on high
spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300)
spectra collected with UVES at ESO VLT. The current measurements were
complemented with Cepheid abundances either provided by our group (75) or
available in the literature, for a total of 439 Galactic Cepheids. Special
attention was given in providing a homogeneous abundance scale for these five
elements plus iron (Genovali et al. 2013, 2014). In addition, accurate
Galactocentric distances (RG) based on near-infrared photometry are also
available for all the Cepheids in the sample (Genovali et al. 2014). They cover
a large fraction of the Galactic thin disk (4.1 <= RG <= 18.4 kpc). We found
that the above five elements display well defined linear radial gradients and
modest standard deviations over the entire range of RG. Moreover, the
[element/Fe] abundance ratios are constant across the entire thin disk; only
the Ca radial distribution shows marginal evidence of a positive slope. These
results indicate that the chemical enrichment history of iron and of the quoted
five elements has been quite similar across the four quadrants of the Galactic
thin disk. The [element/Fe] ratios are also constant over the entire period
range. This empirical evidence indicates that the chemical enrichment of
Galactic Cepheids has also been very homogenous during the range in age that
they cover (~10-300 Myr). Once again, [Ca/Fe] vs. log(P) shows a (negative)
gradient, being underabundant among youngest Cepheids. Finally, we also found
that Cepheid abundances agree quite well with similar abundances for thin and
thick disk dwarf stars and they follow the typical Mg-Al and Na-O correlations.Comment: 25 pages, 11 figures, 5 table
The Carina Project. VIII. The {\alpha}-element abundances
We have performed a new abundance analysis of Carina Red Giant (RG) stars
from spectroscopic data collected with UVES (high resolution) and
FLAMES/GIRAFFE (high and medium resolution) at ESO/VLT. The former sample
includes 44 RGs, while the latter consists of 65 (high) and ~800 (medium
resolution) RGs, covering a significant fraction of the galaxy's RG branch
(RGB), and red clump stars. To improve the abundance analysis at the faint
magnitude limit, the FLAMES/GIRAFFE data were divided into ten surface gravity
and effective temperature bins. The spectra of the stars belonging to the same
gravity/temperature bin were stacked. This approach allowed us to increase by
at least a factor of five the signal-to-noise ratio in the faint limit
(V>20.5mag). We took advantage of the new photometry index cU,B,I introduced by
Monelli et al. (2014), as an age and probably a metallicity indicator, to split
stars along the RGB. These two stellar populations display distinct [Fe/H] and
[Mg/H] distributions: their mean Fe abundances are -2.150.06dex
(sig=0.28), and -1.750.03dex (sig=0.21), respectively. The two iron
distributions differ at the 75% level. This supports preliminary results by
Lemasle et al. (2012) and by Monelli et al. (2014). Moreover, we found that the
old and intermediate-age stellar populations have mean [Mg/H] abundances of
-1.910.05dex (sig=0.22) and -1.350.03dex (sig=0.22); these differ at
the 83% level. Carina's {\alpha}-element abundances agree, within 1sigma, with
similar abundances for field Halo stars and for cluster (Galactic, Magellanic)
stars. The same outcome applies to nearby dwarf spheroidals and ultra-faint
dwarf galaxies, in the iron range covered by Carina stars. Finally, we found
evidence of a clear correlation between Na and O abundances, thus suggesting
that Carina's chemical enrichment history is quite different than in the
globular clusters.Comment: 21 pages, 19 figures, 11 tables, accepted for publication in A&