702 research outputs found
Reaffirming the connection between the Galactic stellar warp and the Canis Major overdensity
We perform a critical re-analysis and discussion of recent results presented
in the literature which interpret the CMa overdensity as the signature of an
accreting dwarf galaxy or a new substructure within the Galaxy. Several issues
are addressed. We show that arguments against the ``warp'' interpretation are
based on an erroneous perception of the Milky Way. There is nothing anomalous
with colour--magnitude diagrams on opposite sides of the average warp mid-plane
being different. We witnessed the rise and fall of the blue plume population,
first attributed to young stars in a disrupting dwarf galaxy and now discarded
as a normal disc population. Similarly, there is nothing anomalous in the outer
thin+thick disc metallicities being low (-1<[Fe/H]<-0.5), and spiral arms (as
part of the thin disc) should, and do, warp. Most importantly, we show
unambiguously that, contrary to previous claims, the warp produces a stellar
overdensity that is distance-compatible with that observed in CMa.The CMa
over-density remains fully accounted for in a first order approach by Galactic
models without new substructures. Given the intrinsic uncertainties (concerning
the properties of the warp, flare and disc cutoff, the role of extinction and
degeneracy), minor deviations with respect to these models are not enough to
support the hypothesis of an accreted dwarf galaxy or new substructure within
the Milky Way disc.Comment: A&A Letter, accepted, 4 pages, 3 figure
Spectroscopic search for binaries among EHB stars in globular clusters
We performed a spectroscopic search for binaries among hot Horizontal Branch
stars in globular clusters. We present final results for a sample of 51 stars
in NGC6752, and preliminary results for the first 15 stars analyzed in M80. The
observed stars are distributed along all the HBs in the range 8000 < Teff <
32000 K, and have been observed during four nights. Radial velocity variations
have been measured with the cross-correlation technique. We carefully analyzed
the statistical and systematic errors associated with the measurements in order
to evaluate the statistical significance of the observed variations. No close
binary system has been detected, neither among cooler stars nor among the
sample of hot EHB stars (18 stars with Teff > 22000 K in NGC6752). The data
corrected for instrumental effects indicate that the radial velocity variations
are always below the 3sigma level of ~15 km/s. These results are in sharp
contrast with those found for field hot subdwarfs, and open new questions about
the formation of EHB stars in globular clusters, and possibly of the field
subdwarfs.Comment: To appear in Baltic Astronomy. Proceedings of the 2nd meeting on Hot
Subdwarf Stars, La Palma, June 2005. 4 pages, 2 figure
The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5
To check the impact of the multiple population scenario for globular clusters
on their HB, we present an analysis of the composition of 110 red HB (RHB)
stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc
we found tight relations between the colours of the stars and their abundances
of p-capture elements. This strongly supports the idea that the He content -
which is expected to be closely correlated with the abundances of p-capture
elements - is the third parameter (after overall metallicity and age) that
determines the colour of HB stars. However, the range in He abundance must be
small (Delta Y<0.03) in 47 Tuc to reproduce our observations; this agrees with
previous analyses. There is possibly a correlation between the abundances of p-
and n-capture elements in 47 Tuc. If confirmed, this might suggest that AGB
stars of moderate mass contributed to the gas from which second-generation
stars formed. Considering the selection effects in our sample (which does not
include stars warmer than 11000 K and RR Lyrae variables) is important to
understand our results for M5. In this case, we find that, as expected, RHB
stars are Na-poor and O-rich, and likely belong to the primordial population.
There is a clear correlation of the [Na/O] ratio and N abundance with colour
along the BHB. A derivation of the He abundance for these stars yields a low
value of Y=0.22\pm 0.03. This is expected because HB stars of a putative
He-rich population in this cluster should be warmer than 11000 K, and would
accordingly not have been sampled by our analysis. However, we need some
additional source of scatter in the total mass loss of stars climbing up the
RGB to reproduce our results for M5. Finally, we found a C-star on the HB of 47
Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These
stars are among the brightest and coolest HB stars.Comment: Accepted on Astronomy and Astrophysics. 16 pages, 19 figure
The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)
We present the abundance analysis of 82 red giant branch stars in the dense,
metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars
analyzed in this way for this cluster. From high resolution UVES spectra of 14
stars and intermediate resolution GIRAFFE spectra for the other stars we
derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean
metal abundance of M 80 is [Fe/H]=-1.791+/-0.006+/-0.076 (+/-statistical
+/-systematic error) with rms=0.023 (14 stars). M 80 shows star to star
variations in proton-capture elements, and the extension of the Na-O
anticorrelation perfectly fit the relations with (i) total cluster mass, (ii)
horizontal branch morphology, and (iii) cluster concentration previously found
by our group. The chemistry of multiple stellar populations in M 80 does not
look extreme. The cluster is also a typical representative of halo globular
clusters for what concerns the pattern of alpha-capture and Fe-group elements.
However we found that a significant contribution from the s-process is required
to account for the distribution of neutron-capture elements. A minority of
stars in M 80 seem to exhibit slightly enhanced abundances of s-process
species, compatible with those observed in M 22 and NGC 1851, although further
confirmation from larger samples is required.Comment: 18 pages, 21 figures, 10 tables; accepted for publication on
Astronomy and Astrophysic
The ubiquitous nature of the Horizontal Branch second U-jump: A link with the Blue Hook scenario?
In a previous paper we reported on a discontinuity in the extreme horizontal
branch (EHB) of the Galactic globular cluster NGC6752, which we called the
second U-jump. This feature was attributed to a combination of post zero-age
horizontal branch evolution and diffusion effects. In this follow-up study we
analyze other EHB clusters and show that the second U-jump is a common feature
among EHB clusters reaching T_{eff}\ge 23,000K, and that its onset in different
clusters converges around T_{eff}\sim 21,000\pm3,000K. We also present
near-ultraviolet diagrams of \omega Cen and NGC2808, the only two objects with
spectroscopically confirmed ``blue hook'' stars (T_{eff}\ge 35,000K). We
confirm predictions of a photometric discontinuity separating late from
early-helium flashers. Moreover, we present empirical evidence that the second
U-jump population might be mainly composed by early-helium flashers. Lastly, we
revisit the discussion on the ubiquitous nature of the gaps and jumps so far
identified in the blue HB tails, suggesting a possible discrete nature of the
distribution in temperature of the HB stars.Comment: 15 pages, 8 figures. Higher resolution version available via ftp at
ftp://ftp.pd.astro.it/pub/momany/momany.tar A&A accepte
NGC 362: another globular cluster with a split red giant branch
We obtained FLAMES GIRAFFE+UVES spectra for both first and second-generation
red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them
to derive abundances of 21 atomic species for a sample of 92 stars. The
surveyed elements include proton-capture (O, Na, Mg, Al, Si), alpha-capture
(Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr,
Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented
for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be
clustered into two discrete groups along the Na-O anti-correlation, with a gap
at [O/Na] 0 dex. Na-rich, second generation stars show a trend to be more
centrally concentrated, although the level of confidence is not very high. When
compared to the classical second-parameter twin NGC 288, with similar
metallicity, but different horizontal branch type and much lower total mass,
the proton-capture processing in stars of NGC 362 seems to be more extreme,
confirming previous analysis. We discovered the presence of a secondary RGB
sequence, redder than the bulk of the RGB: a preliminary estimate shows that
this sequence comprises about 6% of RGB stars. Our spectroscopic data and
literature photometry indicate that this sequence is populated almost
exclusively by giants rich in Ba, and probably rich in all s-process elements,
as found in other clusters. In this regards, NGC 362 joins previously studied
GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).Comment: 16 pages, 23 figures, 11 tables, accepted for publication on
Astronomy and Astrophysic
The extreme chemistry of multiple stellar populations in the metal-poor globular cluster NGC 4833
Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is
extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal
branch (HB). We present the abundance analysis for a large sample of 78 red
giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived
abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y,
Ba, La, Nd. This is the first extensive study of this cluster from high
resolution spectroscopy. On the scale of our survey, the metallicity of NGC
4833 is [Fe/H]=-2.015+/-0.004+/-0.084 dex (rms=0.014 dex) from 12 stars
observed with UVES, where the first error is from statistics and the second one
refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous
at better than 6%. On the other hand, the light elements involved in
proton-capture reactions at high temperature show the large star-to-star
variations observed in almost all GCs studied so far. The Na-O anticorrelation
in NGC 4833 is quite extended, as expected from the high temperatures reached
by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with
extreme [O/Na] ratios. More striking is the finding that large star-to-star
variations are seen also for Mg, which spans a range of more than 0.5 dex in
this GC. Depletions in Mg are correlated to the abundances of O and
anti-correlated with Na, Al, and Si abundances. This pattern suggests the
action of nuclear processing at unusually high temperatures, producing the
extreme chemistry observed in the stellar generations of NGC 4833. This extreme
changes are also seen in giants of the much more massive GCs M 54 and omega
Cen, and our conclusion is that NGC 4833 has probably lost a conpicuous
fraction of its original mass due to bulge shocking, as also indicated by its
orbit.Comment: 18 pages, 16 figures, 11 tables; accepted for publication on
Astronomy and Astrophysic
Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses
We used the ESO VLT-FORS2 facility to collect low-resolution spectra of 51
targets distributed along the Horizontal Branch. We determined atmospheric
parameters by comparison with theoretical models through standard fitting
routines, and masses by basic equations. Results are in general in good
agreement with previous works, although not always with theoretical
expectations for cooler stars (Teff<15000 K). The calculated color excess is
systematically lower than literature values, pointing towards a possible
underestimation of effective temperatures. Moreover, we find two groups of
stars at Teff=14000 K and at Teff=27000$ K that present anomalies with respect
to the general trend and expectations. We suppose that the three peculiar
bright stars at Teff=14000 K are probably affected by an enhanced stellar wind.
For the eight Extreme Horizontal Branch stars at Teff=27000 K which show
unusually high masses we find no plausible explanation. While most of our
results agree well with the predictions of standard horizontal branch
evolution, we still have problems with the low masses we derive in certain
temperature ranges. We believe that Kurucz ATLAS9 LTE model atmospheres with
solar-scaled abundances are probably inadequate for these temperature ranges.
Concerning the group of anomalous stars at Teff=27000 K, a Kolmogorov-Smirnov
test indicates that there is only an 8.4% probability that these stars are
randomly drawn from the general distribution in the color-magnitude diagram.
This is not conclusive but points out that these stars could be both (and
independently) spectroscopically and photometrically peculiar with respect to
the general Extreme Horizontal Branch population.Comment: 12 pages, 10 figures, accepted for pubblication in A&A. Replaced for
typos and better LaTeX outpu
The Na-O anticorrelation in horizontal branch stars. V. NGC 6723
We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high
resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae
variables, and 17 blue horizontal branch (BHB) stars in the low-concentration,
moderately metal-rich globular cluster NGC6723 ([Fe/H]=-1.22+/-0.08 from our
present sample). The spectra were optimized to derive O and Na abundances. In
addition, we obtained abundances for other elements, including N, Fe, Mg, Ca,
Ni, and Ba. We used these data to discuss the evidence of a connection between
the distribution of stars along the horizontal branch (HB) and the multiple
populations that are typically present in globular clusters. We found that all
RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these
stars probably belong to the first stellar generation in this cluster. Only the
four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and
they probably belong to the second generation. While our sample is not fully
representative of the whole HB population in NGC6723, our data suggest that in
this cluster only HB stars warmer than ~9000 K, that is one fourth of the
total, belong to the second generation, if at all. Since in many other clusters
this fraction is about two thirds, we conclude that the fraction of
first/second generation in globular clusters may be strongly variable. In
addition, the wide range in colour of chemically homogeneous first-generation
HB stars requires a considerable spread in mass loss (>0.10 Mo). The reason for
this spread is yet to be understood. Finally, we found a high Ba abundance,
with a statistically significant radial abundance gradient.Comment: Astronomy and Astrophysics, in press; 15 pages; 11 figure
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