2,412 research outputs found
Identification of a Class of Low-Mass Asymptotic Giant Branch Stars Struggling to Become Carbon Stars in the Magellanic Clouds
We have identified a new class of Asymptotic Giant Branch (AGB) stars in the
Small and Large Magellanic Clouds (SMC/LMC) using optical to infrared
photometry, light curves, and optical spectroscopy. The strong dust production
and long-period pulsations of these stars indicate that they are at the very
end of their AGB evolution. Period-mass-radius relations for the
fundamental-mode pulsators give median current stellar masses of 1.14 M_sun in
the LMC and 0.94 M_sun in the SMC (with dispersions of 0.21 and 0.18 M_sun,
respectively), and models suggest initial masses of <1.5 M_sun and <1.25 M_sun,
respectively. This new class of stars includes both O-rich and C-rich
chemistries, placing the limit where dredge-up allows carbon star production
below these masses. A high fraction of the brightest among them should show S
star characteristics indicative of atmospheric C/O ~ 1, and many will form
O-rich dust prior to their C-rich phase. These stars can be separated from
their less-evolved counterparts by their characteristically red J-[8] colors.Comment: 16 pages, 18 figures, accepted for publication in Ap
Matrix exponential via Clifford algebras
We use isomorphism between matrix algebras and simple orthogonal
Clifford algebras \cl(Q) to compute matrix exponential of a real,
complex, and quaternionic matrix A. The isomorphic image in
\cl(Q), where the quadratic form has a suitable signature is
exponentiated modulo a minimal polynomial of using Clifford exponential.
Elements of \cl(Q) are treated as symbolic multivariate polynomials in
Grassmann monomials. Computations in \cl(Q) are performed with a Maple
package `CLIFFORD'. Three examples of matrix exponentiation are given
The first 8-13 micron spectra of globular cluster red giants: circumstellar silicate dust grains in 47 Tucanae (NGC 104)
We present 8-13 micron spectra of eight red giants in the globular cluster 47
Tucanae (NGC 104), obtained at the European Southern Observatory 3.6m
telescope. These are the first mid-infrared spectra of metal-poor, low-mass
stars. The spectrum of at least one of these, namely the extremely red,
large-amplitude variable V1, shows direct evidence of circumstellar grains made
of amorphous silicate.Comment: Accepted for publication in Astronomy and Astrophysics, 5 page
Spitzer Space Telescope evidence in NGC 6791: no super-mass-loss at super-solar metallicity to explain helium white dwarfs?
We use archival Spitzer Space Telescope photometry of the old, super-solar
metallicity massive open cluster NGC 6791 to look for evidence of enhanced mass
loss, which has been postulated to explain the optical luminosity function and
low white dwarf masses in this benchmark cluster. We find a conspicuous lack of
evidence for prolificacy of circumstellar dust production that would have been
expected to accompany such mass loss. We also construct the optical and
infrared luminosity functions, and demonstrate that these fully agree with
theoretical expectations. We thus conclude that there is no evidence for the
mass loss of super-solar metallicity red giants to be sufficiently high that
they can avoid the helium flash at the tip of the red giant branch.Comment: accepted for publication in ApJ Letter
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
Hubble Space Telescope imaging of the compact elliptical galaxy M32 reveals a dearth of carbon stars
We present new {\em Hubble Space Telescope} WFC3/IR medium-band photometry of
the compact elliptical galaxy M32, chemically resolving its thermally pulsating
asymptotic giant branch stars. We find 2829 M-type stars and 57 C stars. The
carbon stars are likely contaminants from M31. If carbon stars are present in
M32 they are so in very low numbers. The uncorrected C/M ratio is 0.020
0.003; this drops to less than 0.007 after taking into account contamination
from M31. As the mean metallicity of M32 is just below solar, this low ratio of
C to M stars is unlikely due to a metallicity ceiling for the formation of
carbon stars. Instead, the age of the AGB population is likely to be the
primary factor. The ratio of AGB to RGB stars in M32 is similar to that of the
inner disc of M31 which contain stars that formed 1.5--4 Gyr ago. If the M32
population is at the older end of this age then its lack of C-stars may be
consistent with a narrow mass range for carbon star formation predicted by some
stellar evolution models. Applying our chemical classifications to the dusty
variable stars identified with {\em Spitzer}, we find that the x-AGB candidates
identified with {\em Spitzer} are predominately M-type stars. This
substantially increases the lower limit to the cumulative dust-production rate
in M32 to 1.97 .Comment: 10 pages, 7 figures, submitted MNRAS 7/12/2
Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal
We present spectra of 1142 colour-selected stars in the direction of the
Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with
VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe
1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at
>99% confidence) for 592 stars on the basis of their radial velocity, and
spectral types are given. Very slow rotation is marginally detected around the
galaxy's major axis. We identify five S stars and 23 carbon stars, of which all
but four carbon stars are newly-determined and all but one (PQ Sgr) are likely
Sgr dSph members. We examine the onset of carbon-richness in this metal-poor
galaxy in the context of stellar models. We compare the stellar death rate (one
star per 1000-1700 years) to known planetary nebula dynamical ages and find
that the bulk population produce the observed (carbon-rich) planetary nebulae.
We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr,
compared to a total thermal-pulsing asymptotic giant branch lifetime of
530-1330 kyr. We conclude by discussing the return of carbon-rich material to
the ISM.Comment: 14 pages, 10 figures, accepted MNRA
Fundamental parameters, integrated RGB mass loss and dust production in the Galactic globular cluster 47 Tucanae
Fundamental parameters and time-evolution of mass loss are investigated for
post-main-sequence stars in the Galactic globular cluster 47 Tucanae (NGC 104).
This is accomplished by fitting spectral energy distributions (SEDs) to
existing optical and infrared photometry and spectroscopy, to produce a true
Hertzsprung--Russell diagram. We confirm the cluster's distance as 4611 (+213,
-200) pc and age as 12 +/- 1 Gyr. Horizontal branch models appear to confirm
that no more RGB mass loss occurs in 47 Tuc than in the more-metal-poor omega
Centauri, though difficulties arise due to inconsistencies between the models.
Using our SEDs, we identify those stars which exhibit infrared excess, finding
excess only among the brightest giants: dusty mass loss begins at a luminosity
of ~ 1000 Lsun, becoming ubiquitous above 2000 Lsun. Recent claims of dust
production around lower-luminosity giants cannot be reproduced, despite using
the same archival Spitzer imagery.Comment: 22 pages, 17 figures, accepted ApJ
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