Initial Metal Abundances Research Articles

The Nature of Anomalous Period Increase in the Pulsating Variable V725 Sgr

The evolutionary tracks of stars with masses on the main sequence $$0.84\;M_{\odot}\leq M_{\textrm{ZAMS}}\leq 0.95\;M_{\odot}$$ and initial metal abundances $$Z=0.006$$ and $$0.01$$ were computed under various assumptions about the mass loss rate at the red giant stage as well as at the AGB and the post-AGB stages. Among 160 evolutionary sequences we selected nearly 30 sequences where the final thermal flash of the helium shell source occurs in the early post-AGB stage when the ratio of the hydrogen envelope mass to the stellar mass ranges from 0.01 to 0.08. Selected evolutionary sequences were used for calculation of initial and inner boundary conditions used in solution of the equations of radiation hydrodynamics and turbulent convection describing evolution of stellar pulsations after the helium flash. Among about three dozen hydrodynamic models we found the three ones demonstrating almost eightfold increase of the pulsation period observed in V725 Sgr during the last century as well as the gradual transformation of fairly regular pulsations with period $$\Pi\approx 12$$ day to semi–regular non-linear oscillations with period $$80\lesssim\Pi\lesssim 90\text{ day}$$ . We conclude that the anomalous growth of the pulsation period in V725 Sgr is due to the final thermal flash of the helium shell source that occurred in the early post-AGB star with mass $$M\approx 0.53\;M_{\odot}$$ and the mass of the hydrogen envelope ranging from $$0.013$$ to $$0.019\;M_{\odot}$$ .

ON THE NECESSITY OF COMPOSITION-DEPENDENT LOW-TEMPERATURE OPACITY IN MODELS OF METAL-POOR ASYMPTOTIC GIANT BRANCH STARS

The vital importance of composition-dependent low-temperature opacity in low-mass (M < 3Msun) asymptotic giant branch (AGB) stellar models of metallicity Z > 0.001 has recently been demonstrated (e.g. Marigo 2002; Ventura & Marigo 2010). Its significance to more metal-poor, intermediate mass (M > 2.5Msun) models has yet to be investigated. We show that its inclusion in lower-metallicity models ([Fe/H] < -2) is essential, and that there exists no threshold metallicity below which composition-dependent molecular opacity may be neglected. We find it to be crucial in all intermediate-mass models investigated ([Fe/H] < -2 and 2.5 < M/Msun < 5), because of the evolution of the surface chemistry, including the orders of magnitude increase in the abundance of molecule-forming species. Its effect on these models mirrors that previously reported for higher-metallicity models - increase in radius, decrease in Teff, faster mass loss, shorter thermally pulsing AGB lifetime, reduced enrichment in third dredge-up products (by a factor of three to ten), and an increase in the mass limit for hot bottom burning. We show that the evolution of low-metallicity models with composition-dependent low-temperature opacity is relatively independent of initial metal abundance because its contribution to the opacity is far outweighed by changes due to dredge-up. Our results imply a significant reduction in the expected number of nitrogen-enhanced metal-poor stars, which may help explain their observed paucity. We note that these findings are partially a product of the macrophysics adopted in our models, in particular the Vassiliadis & Wood (1993) mass loss rate which is strongly dependent on radius.

Absolute dimensions of detached eclipsing binaries -- I. The metallic-lined system WW Aurigae

WW Aurigae is a detached eclipsing binary composed of two metallic-lined A-type stars orbiting each other every 2.5 days. We have determined the masses and radii of both components to accuracies of 0.4 and 0.6 percent, respectively. From a cross-correlation analysis of high-resolution spectra we find masses of 1.964 +/- 0.007 Msun for the primary star and 1.814 +/- 0.007 Msun for the secondary star. From an analysis of photoelectric uvby and UBV light curves we find the radii of the stars to be 1.927 +/- 0.011 Rsun and 1.841 +/- 0.011 Rsun, where the uncertainties have been calculated using a Monte Carlo algorithm. Fundamental effective temperatures of the two stars have been derived, using the Hipparcos parallax of WW Aur and published ultraviolet, optical and infrared fluxes, and are 7960 +/- 420 and 7670 +/- 410 K. The masses, radii and effective temperatures of WW Aur are only matched by theoretical evolutionary models for a fractional initial metal abundance, Z, of approximately 0.06 and an age of roughly 90 Myr. This seems to be the highest metal abundance inferred for a well-studied detached eclipsing binary, but we find no evidence that it is related to the metallic-lined nature of the stars. The circular orbit of WW Aur is in conflict with the circularization timescales of both the Tassoul and the Zahn tidal theories and we suggest that this is due to pre-main-sequence evolution or the presence of a circular orbit when the stars were formed.

Evolved Massive Stars in the Local Group. II. A New Survey for Wolf‐Rayet Stars in M33 and Its Implications for Massive Star Evolution: Evidence of the “Conti Scenario” in Action

We expect the evolution of massive stars to be strongly influenced by mass loss and hence to be sensitive to metallicity. It should be possible to test this "Conti scenario" be comparing the populations of evolved massive stars among the Local Group galaxies, but such investigations have been hampered by incompleteness. In a previous paper, we presented results of a new survey for red supergiants (RSGs) in selected regions of the Local Group galaxies M33, M31, and NGC 6822. In the present paper, we survey eight fields in M33 for Wolf-Rayet stars (WRs), using interference-filter imaging with a CCD to select candidates. Follow-up spectroscopy is used to confirm 22 newly found WR stars, 21 of WN type. We establish that our survey would readily detect WRs as weak-lined as any known, and we conclude that our survey is essentially complete. This survey confirms suspicions that the previous photographic surveys were only 50% complete for WN-type WRs and allows us to combine the data with equally complete samples on other Local Group galaxies. We find that the relative number of WC- and WN-type WRs correlates extremely well with metallicity, varying by a factor of 3 with galactocentric distance within the plane of M33, and continuing the trend to lower and higher metallicity galaxies. The WC/WN ratio within 3 kpc of the sun is slightly above this trend, and we argue that WN stars are underrepresented in this sample. The WC/WN ratio is anomalously high in IC 10, given its low metallicity, and we demonstrate that this is not because of selection effects but is likely caused by IC 10's current status as a starburst system. We examine the spectral properties of WC stars within these galaxies, confirming the previously reported trends that the spectral lines are stronger and broader in regions of lower metallicity. We suggest that the different WC spectral subclasses do not primarily indicate different physical properties for these stars but rather are simply a reflection of the effect that the initial metal abundances has had on the stellar wind structure. Finally, we compare the luminous RSGs with WRs in these galaxies. We find that there is a very strong correlation of the relative numbers of RSGs and WRs with metallicity, in the sense predicted by Maeder, Lequeux, & Azzopardi: at lower metallicities the fraction of luminous (Mbol < -7) RSGs is higher, with a factor of 6 change within the disk of M33 [Δ log (O/H) = 0.35 dex], and a factor of ~10 change from M31 (or the inner portions of M33) to NGC 6822 [Δ log (O/H) = 0.5 dex]. This is easily explained by the Conti scenario in terms of massive stars spending proportionately less of their He-burning lifetimes as RSGs rather than WRs at higher metallicities and hence higher mass-loss rates. Finally, we note that the presence of luminous RSGs and WRs stars is extremely well correlated for the OB associations in M31 and M33: where one finds one, one finds the other. To the extent that an association is strictly coeval, this argues that some stars of 15 M☉ and above indeed do go through both RSG and WR stages. The presence of WR stars of both WN and WC types in the same associations as luminous RSGs further suggests that some WCs, at least, have gone through the RSG phase. We include an Appendix providing a complete catalog of confirmed WR stars in Local Group galaxies beyond the Magellanic Clouds.

Analysis of Three Fast Oxygen‐Neon‐Magnesium Novae

Since astronomers have only known about the existence of oxygen-neon-magnesium (ONeMg) novae for the last decade, little work has been done on them as a group. The purpose of this work was to study three fast ONeMg novae: V838 Nova Herculis 1991 (Her 91), V693 Nova Coronae Australis 1981 (CrA 81), and Nova LMC 1990 No. 1 (LMC 90a). Each of these novae has been studied in detail, and a comparison of the results is provided in order to better understand ONeMg novae as a class. My analyses have provided values of the elemental abundances, ejection velocities, ejected masses, white dwarf (WD) masses, maximum outburst luminosities, and white dwarf turn-off times for these novae. All three novae have abundances enhanced relative to solar values, with the oxygen abundance of Her 91 being the exception (see Table 1). They show high N/O ratios, indicating high-mass WD progenitors. We find CrA 81 and LMC 90a to have almost identical abundances, even though the initial metal abundances for the LMC novae are one-third of solar values. Each of the novae exhibited a super-Eddington luminosity phase at maximum light. Our analysis of the Her 91 spectrum taken 2 months after outburst, however, required a luminosity almost 2 orders of magnitude lower than the Eddington limit for a 1.4 M, WD. The best fits for CrA 81 and LMC 90a, on the other hand, were consistent with a constant luminosity of ∼1038 ergs s for at least 2 months after outburst. While we were unable to accurately determine ejected masses for either CrA 81 or LMC 90a owing to the lack of optical data, we found an ejected mass of ∼1024 M, for Her 91. This is in agreement with the values found by other groups using different data and different techniques. The results of this study can be used in many ways to help further the understanding of novae and stellar evolution. The elemental abundances that have been determined can be used as input to theoretical hydrodynamic models. This will provide better constraints on the models in order to determine the energetics of the outburst and the mass of the underlying white dwarf, properties that are difficult to obtain observationally. Knowledge of the abundances will also help determine the contribution of fast ONeMg novae to the heavy-element content of the interstellar medium. In particular, with the abundance results, the mass fraction of Al in the nova ejecta can be estimated. These estimates will help to determine if novae are important for Galactic nucleosynthesis. Analysis of LMC 90a provides the means to compare Galactic with extragalactic novae. For example, the LMC novae can be used to calibrate the luminosities of (and therefore distances to) Galactic novae. This calibration will allow a better determination of the fast end of the absolute-magnitude decay-rate relationship or perhaps develop a separate relationship for this class of novae.

The dwarf spheroidal galaxy in Draco. IV - On the mean metal abundance and metal-abundance range of the red giant stars

view Abstract Citations (17) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The dwarf spheroidal galaxy in Draco. IV - On the mean metal abundance and metal-abundance range of the red giant stars Stetson, P. B. Abstract The spectrophotometry of Zinn (1978) and of Searle and Zinn (1978) is reexamined in order to determine whether the range of spectral properties at constant luminosities among Draco's giants is better explained by a range of initial metal abundance or by the presence of stars in different evolutionary states. Also investigated is whether Draco's giants are sufficiently similar to those in the globular clusters (GCs) such that Searle and Zinn's empirical technique for determining relative metal abundance is directly applicable. It is concluded that Draco probably differs from the clusters in some important, discrete manner; thus, secondary observational parameters which are correlated with metal abundance in normal CGs may not accurately predict the metallicity of Draco. It is also found that there is indeed a spread of temperature and spectral properties on Draco's upper giant branch, but they resemble the differences found between the first-time giant and asymptotic giant stars in GCs and do not necessarily imply a range of initial metal abundance. Publication: The Astronomical Journal Pub Date: April 1980 DOI: 10.1086/112688 Bibcode: 1980AJ.....85..398S Keywords: Abundance; Metallic Stars; Red Giant Stars; Spiral Galaxies; Stellar Spectra; Stellar Spectrophotometry; Galactic Clusters; Globular Clusters; Hertzsprung-Russell Diagram; Spectral Bands; Stellar Evolution; Stellar Luminosity; Stellar Temperature; Astrophysics full text sources ADS | data products NED (1)

Chemical evolution of the galaxy during its contraction

Models for the chemical evolution of the galaxy are constructed in which the time evolution is imposed by the contraction rate of the galaxy and present observations of stellar metal abundances as a function of height above the galactic plane. Stars with massm≲3.5m ⊙ do not contribute to the metal enrichment of the interstellar gas, and we argue that the interstellar metal abundance at this epoch should be fairly insensitive to the size of the mass fraction of the galaxy that condenses into such stars. The birth rate for stars more massive than 3.5m ⊙ is assumed proportional toV gal − , whereV gal is the contracting volume of the main body of the galaxy. If a dynamic time-scale is adopted for the contraction of the galaxy, then the assumed power-law birth rate yields suitable chemical evolution models only if observed Population II metals are synthesized in stars more massive than about 8.5m ⊙. This mass range is consistent with the predictions of current stellar-evolution theory. Provided the birth function does include stars more massive than 8.5m ⊙, the relation between the value of the parametern in the birth rate and the observed chemical evolution rate is not particularly sensitive to the specific form adopted for the initial mass spectrum, or to the proportionality constant in the birth rate. We find (i)n≲1.4, in general, and (ii)n is close to 1.4 if the contraction of the galaxy to a heighth=400 pc above the plane occurs at close to the free-fall rate. These results are independent of the form of the initial mass spectrum, providedS 3 is small. HereS is the total mass fraction of the galaxy that cycles through stars during its contraction. Numerical models, with an explicit initial mass spectrum, indicate that the same restrictions on the values ofn apply approximately whenS 3 is not small. To introduce low mass stars, we allow the birth rate for stars more massive than 3.5m ⊙ to level off at a time intervalt L just before the contraction of the galaxy stops, while the total birth rate remains a simple power law. We find that reasonable models are obtained witht L ⋟1.5×107 yr if the galaxy contracts at a dynamic rate. However, aside from these restrictions on the values ofn andt L , there is no uniquely favored model. For any suitable model, the supernova rate must be small enough so that shock waves from neighboring supernovae do not collide during the adiabatic expansion stage. Otherwise, the interstellar gas would not have time to cool, and its high temperature would tend to impede both star formation and the rapid contraction of the galaxy. The supernova rates in the numerical models given here are small enough to avoid this problem, but large enough to achieve a uniform metal abundance on a time scale short compared to the chemical-evolution time scale. At the epoch considered here, the interstellar metal abundance is approximately less than 0.4Z ⊙, and the models are assumed to apply before galactic-scale inhomogeneities, such as the galactic nucleus, become important. Therefore, the chemical mixing time scales imply that most Population II stars of the same age should have approximately the same initial metal abundance, unless the clustering of supernova explosions associated with massive Population II stars is significant. It is shown that collisions between shock waves from neighboring supernovae can produce local regions of significantly enhanced density. The peak bolometric luminosity of the galaxy during its contraction is similar to that predicted by Partridge and Peebles (1967a), but it occurs during the final stages of contraction to the disc. Numerical models give values between 13 and 34 yr−1 for the average number of supernova explosions per year during this bright phase. The X-ray luminosity of the galaxy from these supernovae may be comparable to that of Seyfert galaxies.