Hydrogen-burning Main Sequence Research Articles

Probing the inner Galactic halo with blue horizontal-branch stars

Context. Stars that are found on the blue horizontal-branch (BHB) evolved from low-mass stars that have completed their core hydrogen-burning main sequence (MS) stage and undergone the helium flash at the end of their red giant phase. Hence, they are very old objects that can be used as markers in studying galactic structure and formation history. The fact that their luminosity is virtually constant at all effective temperatures also makes them good standard candles. Aims. We have compiled a catalogue of BHB stars with stellar parameters calculated from spectral energy distributions (SEDs) constructed using data from multiple large-scale photometric surveys. In addition, we update our previous Gaia Early Data Release 3 (EDR3) catalogue of BHB stars with parallax errors less than 20% by using the SED results to define the selection criteria. The purpose of these catalogues is to create a set of BHB star candidates with reliable stellar parameters. In addition, they provide a more complete full-sky catalogue with candidate objects found along the whole BHB from where RR-Lyrae are found on the instability strip to the extreme horizontal-branch (EHB). Methods. We selected a large dataset of Gaia Data Release 3 (DR3) objects based only on their position on the colour-magnitude diagram (CMD), along with the tangential velocity and parallax errors. The SEDs were then used to evaluate contamination levels in the dataset and derive optimised data quality acceptance constraints. This allowed us to extend the Gaia DR3 colour and absolute magnitude criteria further towards the EHB. The level of contamination found using SED analysis was confirmed by acquiring spectra using the Ondrejov Echelle spectrograph, attached to the Perek 2m telescope at the Astronomical Institute of the Czech Academy of Sciences. Results. We present a catalogue of 9172 Galactic halo BHB candidate stars with atmospheric and stellar parameters calculated from synthetic SEDs. We also present an extended Gaia DR3-based catalogue of 22 335 BHB candidate stars with a wider range of effective temperatures and Gaia DR3 parallax errors of less than 20%. This represents an increase of 33% compared to the our 2021 catalogue, with a contamination level of 10%.

Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

We report the discovery of five transiting companions near the hydrogen-burning mass limit in close orbits around main sequence stars originally identified by the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213. Using TESS and ground-based photometry as well as radial velocities from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the companions have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 MJup , and radii between 0.81 and 1.66 RJup . These targets have masses near the uncertain lower limit of hydrogen core fusion (~73-96 MJup ), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 ± 0.1 Gyr) and TOI-681 (0.17 ± 0.03 Gyr) and found them to have relatively larger radii compared to other transiting companions of a similar mass. Conversely we estimated older ages for TOI-148 and TOI-746 and found them to have relatively smaller companion radii. With an effective temperature of 9800 ± 200 K, TOI-587 is the hottest known main-sequence star to host a transiting brown dwarf or very low-mass star. We found evidence of spin-orbit synchronization for TOI-148 and TOI-746 as well as tidal circularization for TOI-148. These companions add to the population of brown dwarfs and very low-mass stars with well measured parameters ideal to test formation models of these rare objects, the origin of the brown dwarf desert, and the distinction between brown dwarfs and hydrogen-burning main sequence stars.

Analytic Models of Brown Dwarfs and the Substellar Mass Limit

We present the analytic theory of brown dwarf evolution and the lower mass limit of the hydrogen burning main-sequence stars and introduce some modifications to the existing models. We give an exact expression for the pressure of an ideal nonrelativistic Fermi gas at a finite temperature, therefore allowing for nonzero values of the degeneracy parameter. We review the derivation of surface luminosity using an entropy matching condition and the first-order phase transition between the molecular hydrogen in the outer envelope and the partially ionized hydrogen in the inner region. We also discuss the results of modern simulations of the plasma phase transition, which illustrate the uncertainties in determining its critical temperature. Based on the existing models and with some simple modification, we find the maximum mass for a brown dwarf to be in the range0.064M⊙–0.087M⊙. An analytic formula for the luminosity evolution allows us to estimate the time period of the nonsteady state (i.e., non-main-sequence) nuclear burning for substellar objects. We also calculate the evolution of very low mass stars. We estimate that≃11% of stars take longer than107 yr to reach the main sequence, and≃5% of stars take longer than108 yr.

A COMPACT DEGENERATE PRIMARY-STAR PROGENITOR OF SN 2011fe

While a white dwarf is, from a theoretical perspective, the most plausible primary star in Type Ia supernova (SN Ia), many other candidates have not been formally ruled out. Shock energy deposited in the envelope of any exploding primary contributes to the early SN brightness and, since this radiation energy is degraded by expansion after the explosion, the diffusive luminosity depends on the initial primary radius. We present a new non-detection limit of the nearby SN Ia 2011fe, obtained what appears to be just 4 hours after explosion, allowing us to directly constrain the initial primary radius, R_p. Coupled with the non-detection of a quiescent X-ray counterpart and the inferred synthesized Ni mass, we show that R_p <~ 0.02 R_sun (a factor of 5 smaller than previously inferred), that the average density of the primary must be rho_p > 10,000 gm cm^{-3}, and that the effective temperature must be less than a few x 10^5 K. This rules out hydrogen burning main sequence stars and giants. Constructing the helium-burning main sequence and carbon-burning main sequence, we find such objects are also excluded. By process of elimination, we find that only degeneracy-supported compact objects---WDs and neutron stars---are viable as the primary star of SN 2011fe. With few caveats, we also restrict the companion (secondary) star radius to R_ c <~ 0.1 R_sun, excluding Roche-Lobe overflowing red giant and main-sequence companions to high significance.

White dwarfs as astrophysical probes

AbstractMuch of our knowledge regarding the ages of stars derives from our understanding of the Hertzsprung-Russell Diagram. The diagram is typically dominated by hydrogen burning main-sequence stars, which historically, have been used to establish our most fundamental knowledge of stellar ages and evolution. In this brief article, I highlight how deep ground and space based imaging can uncover the stellar remnants of these hydrogen burning stars, white dwarfs. We have followed up our initial discovery of several large white dwarf populations in nearby star clusters with multiobject spectrographs. The spectroscopy allows us to characterize the properties of the remnant stars (e.g., mass, temperature, and age), which are in turn used to shed new light on fundamental astrophysical problems. Specifically, we estimate the ages of the Milky Way disk and halo, provide the inputs needed to calculate the chemical evolution of galaxies, and re-iterate the important role of HB stars in producing the UV-upturn seen in elliptical galaxies.

The VLT FLAMES Survey of Massive Stars: Rotation and Nitrogen Enrichment as the Key to Understanding Massive Star Evolution

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg, and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ~300 km s-1 using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen-enriched intrinsic slow rotators (vsin i 50 km s-1) incompatible with our models (~20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (log g < 3.7 dex) fast rotators that are relatively unenriched (a further ~20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen-burning main sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields is required to understand the surface properties of a population of massive main-sequence stars.

Hubble Space TelescopeObservations of the Main Sequence of M4

We present new results from a photometric study of the main-sequence stars in M4 (NGC 6121=C1620-264), the closest globular cluster to the Sun. Multifield, multiepoch observations at approximately 1, 2, and 6 core radii were obtained with WFPC2 on the Hubble Space Telescope through either the F606W filter or the F555W (V) and F814W (I) filters. The multiepoch observations allowed us to clean the data on the basis of proper motion and thus separate cluster from field stars or extragalactic objects. In all the fields the cluster main sequence can be traced to at least V = 27.0, but there remains a trail of stars to the limit of the data near V = 30 in the deepest outer field. There is no evidence that we have reached the end of the hydrogen-burning main sequence in any of our fields; however, there is some indication that very few stars remain to be detected in the deepest data. A study of the scatter about the cluster main sequence yields a surprisingly small and variable binary fraction: fb 2% in the inner parts of the cluster, falling to the 1% range outside. However, with one possible exception, no stars in the 6 core radius field exhibit photometric variability on timescales of a few hours through a few days. For the currently visible main-sequence stars, the cluster mass function (MF) is very flat (? = 0.1) in the outer field and flattens further in the inner fields, suggesting well-developed mass segregation. The observed variation in the MF is broadly consistent with isotropic, multimass Michie-King models. Because we have a large sample of white dwarfs in the outer field, we are able to show that the cluster IMF above 0.8 M? was considerably steeper than the present-day MF for low-mass stars. Two appendices are included in this contribution. The first is a detailed discussion of the techniques used to reduce the data, while the second provides a direct comparison between the cluster stars and those belonging to the inner spheroid of the Galaxy. This yields a relative distance between the cluster, dc, and the Galactic center, Ro, of Ro/dc = 4.36 ? 0.13. With our subdwarf-based estimate of dc = 1.73 ? 0.09 kpc to M4, we find Ro = 7.5 ? 0.6 kpc.

The present-day chemical composition of the SMC from UVES spectra of the sharp-lined, B-type dwarf AV 304

High-resolution spectroscopic VLT/UVES observations are presented for the B-type main-sequence star, AV 304, in the Small Magellanic Cloud (SMC). These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As AV 304 is located within the hydrogen burning main-sequence band, its chemical composition should reflect that of the SMC interstellar medium (ISM). A detailed line-by-line differential analysis has been undertaken relative to a Galactic comparison star. A general metal deficiency for the a-process elements O, Si & S of -0.43 +/- 0.05 dex is found for AV 304, with iron having a similar underabundance. Oxygen may be relatively over-abundant by similar to0.1 dex and carbon and aluminium underabundant by similar to0.2 dex. A large nitrogen underabundance (of -1.2 dex relative to hydrogen and -0.7 dex relative to iron) is found. This is interpreted in terms of the CNO bi-cycle having been suppressed in the SMC. Furthermore, the large nitrogen deficiency is in excellent agreement with that found for SMC H II regions. Indeed, this represents a first for stellar astrophysics - confirming the low base-line nitrogen composition of the SMC ISM (viz. 12+log(N/H) similar to 6.66 +/- 0.10 dex).

The present-day chemical composition of the LMC

High-resolution observations of five OB-type main-sequence stars in the Large Magellanic Cloud (LMC) have been obtained with the UCLspectrograph on the 3.9-m Anglo-Australian Telescope. These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As these stars are located within the hydrogen burning main-sequence band, their surface abundances should reflect those of the present-day interstellar medium. Detailed line-by-line differential analyses have been undertaken relative to Galactic comparison stars. We conclude that there exists a general metal deficiency of −0.31 ± 0.04 dex within the LMC, and find no significant abundance variations between cluster and field stars. There is also tentative evidence to suggest a lower oxygen to iron abundance ratio, and an over-deficiency of magnesium relative to the other α-elements. These are discussed in terms of previous abundance analyses and models of discontinuous (or bursting) star formation within the LMC. Finally, there is some evidence to suggest a greater chemical enrichment of material within the H  region LH104.

On the Evolution of Stars that Form Electron‐degenerate Cores Processed by Carbon Burning. V. Shell Convection Sustained by Helium Burning, Transient Neon Burning, Dredge‐out, Urca Cooling, and Other Properties of an 11M⊙Population I Model Star

A stellar model of mass 11 M☉ and Population I composition is evolved from the hydrogen-burning main sequence through the core carbon-burning phase. In contrast with 9, 10, and 10.5 M☉ models studied in earlier papers of this series, carbon burning is ignited at the center of the 11 M☉ model. Like the 10.5 M☉ model, the 11 M☉ model experiences a dredge-out episode at the end of the carbon-burning phase. At the beginning of this episode, a semiconvective zone forms at the base of the hydrogen-rich envelope and carries hydrogen inward in mass toward the outer edge of a fully convective zone that is sustained by helium burning at its base. Hydrogen diffuses into the helium-rich convective zone untila hydrogen shell flash occurs. Helium burning dies out and the outer edge of the convective layer,sustained by fluxes due to hydrogen burning, extends outward in mass through hydrogen-rich material, mixing freshly synthesized nuclei outward. Then, hydrogen burning dies out and the outer edge of the convective shell, now sustained primarily by fluxes due to the release of gravothermal energy, moves outward until it reaches the inner edge of the convective envelope. Freshly synthesized material is then convected to the surface. Mixing during the final phase of homogenization in the convective envelope is maintained by fluxes due to the release of gravothermal energy. At the end of the dredge-out phase, the surface nitrogen abundance has decreased and the C/N ratio has changed from less than unity to larger than unity, showing that mixing has extended into regions where helium burning has manufactured substantial quantities of 12C and destroyed 14N. Prior to the dredge-out phase, neon burning is narrowly averted, and, after the dredge-out phase, neutrino losses due to electron capture and decay reactions between A=25 and A=23 isotopes in and above convective Urca shells cool the inner portions of the electron-degenerate oxygen-neon (ONe) core. Ultimately, the model becomes a thermally pulsing super-asymptotic giant branch (TPSAGB) star with an ONe core of mass ~1.368 M☉. Hydrogen and helium burning over a period of ~1.4×104 yr of TPSAGB evolution add a carbon-oxygen layer of mass ~0.014 M☉ to the electron-degenerate core. Then, electron captures on products of carbon burning lead to the collapse of the core into a neutron star and expulsion of the envelope in a weak Type II supernova explosion. The ratio of helium to hydrogen in the ejecta is approximately twice solar.

High-resolution spectra of very low-mass stars

We present the results of high-resolution (1-0.4A) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I-K > 3.5 (MBol > 12.8) and those of more massive M-dwarfs (MBol = 7-10). A spectral atlas at high (0.4A) resolution for M8-M9+ stars is provided, and the equivalent widths of CsI, RbI and Halpha lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of ``inactive, rapid rotators''. Such objects seem to be common at and below the hydrogen burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism which heats the chromosphere, or the mechanism which generates magnetic fields, is greatly suppressed.

On the Evolution of Stars that Form Electron‐Degenerate Cores Processed by Carbon Burning. III. The Inward Propagation of a Carbon‐Burning Flame and Other Properties of a 9M☉Model Star

A 9 M☉ stellar model of Population I composition is evolved from the hydrogen-burning main sequence to the thermally pulsing super asymptotic giant branch stage, where it has an electron-degenerate core composed of an inner oxygen-neon (ONe) part of mass ~1.066 M☉ and an outer carbon-oxygen (CO) layer of mass ~0.05 M☉ and is experiencing thermal pulses driven by helium-burning thermonuclear flashes. The carbon-burning phase of the 9 M☉ model is in many respects similar to, but differs importantly from that of a 10 M☉ model studied earlier. In both cases, carbon is ignited off center, and a series of carbon flashes accompanied by a convective shell occur. In contrast to the 10 M☉ model, the 9 M☉ model experiences the second dredge-up phenomenon (the penetration of the base of the hydrogen-rich convective envelope inward into helium- and carbon-rich material) near the beginning rather than near the end of the carbon-burning phase. The first carbon-burning flash causes helium burning to shut down and the release of gravothermal energy (compressional and thermal energy) between the helium-carbon discontinuity and the base of the convective envelope plays a dominant role in the dredge-up event. Beginning with the third carbon-burning shell flash, the defined as being coincident with the base of the convective shell, propagates inward with a speed close to theoretical predictions that relate speed to local thermodynamic, opacity, and energy-generation rate characteristics. Ahead of the inward moving front, most of the nuclear energy released in a flame goes into heating and expanding matter. As the precursor moves toward the center, its radial thickness decreases and, to follow the progress of the front with standard techniques, both the spatial grid size and the time step must be continually decreased. Following the front gives one the opportunity to ponder Zeno's paradox, which is averted because the thickness of the precursor remains finite. On reaching the center, the carbon-burning reverses direction and continues moving outward until it is within ~0.03 M☉ of the helium-burning shell. After carbon burning is completed,12C remains at a finite abundance throughout the electron-degenerate core of mass ~1.116 M☉ and is more abundant than 20Ne in the outer ~0.05 M☉ of this core. Over most of the ONe interior of both the 9 and 10 M☉ models,23Na is more abundant than 24Mg, but the maximum 12C abundance in the 9 M☉ model ONe interior (X[12C] ~ 0.048) is significantly larger than in the 10 M☉ model (X[12C] ~ 0.012). For an ONe white dwarf that accretes enough matter to reach the Chandrasekhar limiting mass, this may make the difference between total explosive disruption (large 12C abundance) and collapse to neutron-star dimensions (small 12C abundance). The abundances in the CO part of the core have relevance for understanding the abundances in the ejecta of classical novae produced by massive ONe white dwarfs in close binaries. In the outer ~0.014 M☉ of the CO part of the core, the abundances of all neon isotopes are much less than solar, and 25Mg and the neutron-rich isotopes made during the formation of 25Mg are at a total abundance equal to the initial abundance of CNO elements in the model. As in the 10 M☉ case, thermal pulses occasioned by helium shell flashes begin after hydrogen is reignited and the carbon-burning luminosity drops below ~100 L☉. The time between pulses is ~400 yr, roughly twice as large as in the 10 M☉ model. After the ejection of the hydrogen-rich envelope as a planetary nebula, the remnant of the 9 M☉ model is expected to evolve into a white dwarf of mass ~1.15 M☉, the outer ~0.08 M☉ of which is composed of carbon and oxygen.

On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M[SUB]sun[/SUB] Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse

On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M[SUB]sun[/SUB] Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse

Very low mass stars and white dwarfs in NGC 6397

Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m<SUB>814</SUB> approximately = m<SUB>I</SUB> approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m<SUB>814</SUB> = 18.5 to m<SUB>814</SUB> = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m<SUB>814</SUB> approximately = 20 and m<SUB>814</SUB> approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M<SUB>814</SUB> approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity function with its theoretical counterpart.

Low Mass Stars and White Dwarfs in NGC 6397

Deep WFPC2 images in wide bands centered at 606 and 802 nm were taken with the HST 5.6 arcminutes from the center of the galactic globular cluster NGC 6397. The images were used to accurately position ~ 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m814 ≃ mI ≃ 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m814 = 18.5 to m814 = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m814 ≃ 20 and m814 ≃ 22.5 are evident. The corresponding luminosity function increases slowly from M814 ≃ 6.5 to 8.5 as expected from ground based observations but then drops sharply from there down to the measurement limit. The corresponding mass function obtained by using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between ~ 0.25 and~ 0.15 M⊙, but drops towards the expected mass limit of the normal hydrogen burning main sequence at about 0.1 M⊙. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 M⊙ at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition.

On the formation and evolution of super-asymptotic giant branch stars with cores processed by carbon burning. 1: SPICA to Antares

view Abstract Citations (104) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Formation and Evolution of Super--Asymptotic Giant Branch Stars with Cores Processed by Carbon Burning. I. SPICA to Antares Garcia-Berro, Enrique ; Iben, I. Abstract A 10 solar mass model of Population I composition is evolved from the hydrogen-burning main sequence onto the early 'super'-asymptotic giant branch (ESAGB), where hydrogen does not burn, the dominant source of surface luminosity is shell helium burning, and carbon burning in the helium-exhausted core converts all of the initial C-12 and Ne-22 and some of the O-16 in this core into isotopes of oxygen, neon, sodium, magnesium, and other heavy elements. The entire ESAGB phase lasts approximately 4.6 x 104 yr, and helium burning is the primary source of surface luminosity for all but approximately 3000 yr. The carbon-burning phase lasts approximately 2 x 104 yr. In comparison, the main-sequence phase lasts approximately 2 x 107 yr, and the core helium-burning phase lasts approximately 4 x 106 yr. At the end of the core carbon-burning phase, dredge-up decreases the mass of the hydrogen-exhausted core from approximately 2.46 solar mass to approximately 1.315 solar mass. During the dredge-up episode, in addition to products of complete hydrogen burning, products of incomplete helium burning are mixed into the hydrogen-rich convective enevelope. At the end of the second dredge-up phase, the model has an electron-degenerate carbon-exhausted core of mass approximately 1.285 solar mass. Between the carbon-exhausted core and the hydrogen-rich convective envelope are three layers which are each of mass approximately 0.01 solar mass and which contain (1) C-12 and O-16 at comparable abundances by mass; (2) C-12 and He-4 at comparable abundances by mass; and (3) mostly He-4 with some C-12. The abundances by mass of the main constituents of the carbon-exhausted core vary signifcantly through the core. At the center, abundances of mass are X(O-16) approximately 0.80, X(Ne-20) approximately 0.11, X(Na-22) approximately 0.03, X(Mg-24) approximatelly 0.0075, and X(Mg-26) approximately 0.01, whereas, near the edge of the core, X(O-16) approximately 0.19, X(Ne-20) approximately 0.50, X(Na-23) approximately 0.04, and X(Mg-24) approximately 0.27. The abundance of Na-23 is large enough that, if the mass of the SAGB core can grow to approximately 1.37 solar mass, core collapse into a neutron star will be initiated by electron capture on Na-23 rather than on Mg-24. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174729 Bibcode: 1994ApJ...434..306G Keywords: Asymptotic Giant Branch Stars; Carbon; Helium; Nuclear Fusion; Star Formation; Stellar Composition; Stellar Cores; Stellar Evolution; Stellar Models; Abundance; Main Sequence Stars; Red Giant Stars; Stellar Luminosity; Astrophysics; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES; STARS: AGB AND POST-AGB; STARS: EVOLUTION; STARS: INTERIORS; STARS: SUPERGIANTS full text sources ADS | data products SIMBAD (2) Related Materials (4) Part 2: 1996ApJ...460..489R Part 3: 1997ApJ...485..765G Part 4: 1997ApJ...489..772I Part 5: 1999ApJ...515..381R

An optical study of the faint end of the stellar luminosity function

We implement a new method by which to study the faint end of the field star luminosity function. The method relies on deep, multicolor photometry of fields projected against highly obscured, nearby molecular clouds. The clouds act as nearby opaque screens and delimit a well-defined survey volume which is in principle free of the problem of distinguishing nearby, intrinsically faint dwarf stars from more distant red giants. This study is based upon deep photographic and CCD photometry at optical (V, R, I) bandpasses toward the most highly obscured portions of the Taurus and Ophiuchus molecular clouds. The total volume delimited by the clouds is approximately 200 cu pc. Within this region our survey is complete for all stars brighter than M<SUB>V</SUB> = 16-17 mag; at R and I, the survey is complete down to the lowest mass stars capable of sustaining core hydrogen burning. We estimate the faint end of the field star luminosity function for the composite Taurus and Ophiuchus foreground sample and find that it resembles the local luminosity function down to M<SUB>V</SUB> approx. 16. At still fainter magnitudes we find more stars than do photometric parallex studies of the polar regions. This difference widens dramatically if even the simplest correction for incompleteness is applied to our data. We therefore tentatively conclude that the luminosity function rises beyond M<SUB>V</SUB> approx. 16; even if we discard our attempts to correct for incompleteness in the faintest magnitude bins, the luminosity function at least remains flat for the lowest mass stars. Our provisional finding that the luminosity function rises beyond its well-known peak at M<SUB>V</SUB> approx. 12-13, implies that the initial mass function (IMF) probably rises beyond the turnover point associated with this peak. Even if our most conservative estimate for the faint end of the luminosity function is used-in which no corrections are made for incompleteness-the IMF must at least remain flat down to the edge of the hydrogen-burning main sequence.

Our Sun. III. Present and Future

Self-consistent evolutionary models were computed for our Sun, using Los Alamos interior opacities and Sharp molecular opacities, starting with contraction on the Hayashi track, and fitting the observed present solar L, R, and Z/X at the solar age. This resulted in presolar Y=0.274 and Z=0.01954, and in present solar 37 Cl and 71 Ga neutrino capture rates or 6.53 and 123 SNU, respectively. We explored the Sun's future. While on the hydrogen-burning main sequence, the Sun's luminosity grows from 0.7 L ○. , 4.5 Gyr ago, to 2.2 L ○. , 6.5 Gyr from now

The Faint End of the Stellar Luminosity Function

The observed stellar luminosity function (SLF) of field stars near the Sun is probably the most basic of data concerning stars in the Galaxy; the SLF is defined as the number of stars per cubic parsec within an absolute magnitude interval M; ± bM; where j denotes either bolometric or a specific bandpass and typically bM; = 0.5 mag. It forms the basis for calculating the observed mass densities for a variety of stars thus enabling the deri· vation of the total observed stellar mass densities for comparison with dynamically derived mass densities. It is also used to reconstruct the initial mass function (IMF) for faint stars and to explore the star formation history of the disk and halo. It is of great interest to measure stellar luminosity functions for different subgroups of stars: for main sequence stars with different metallicities, for degenerate white dwarfs, and for globular clusters, open clusters, and active star forming regions (T·associations). Comparison of these lumin· osity functions provides information about many different aspects of astronomy such as star formation scenarios, stellar evolution, cooling theories of white dwarfs, the evolution of stellar populations in galaxies, and the formation of galaxies by the disruption of clusters. In this review we summarize the most recent measurements of the SLFs below Mv = 5; this regime includes hydrogen burning main sequence stars with masses between � 0.08 M 0 and I M 0, substellar objects with masses below the mimimum main· sequence mass (i.e. the brown dwarfs, hereafter

The relation of Algols and W Serpentis stars

Evidence on the issues of whether the W Serpentis stars are a coherent class, and how they may interface with the Algol systems, is reviewed, with emphasis on the idea that they are semi-detached systems in the latter part of the rapid phase of mass transfer, with optically and geometrically thick disks of transferred gas around the (now) more massive star. We are interested in what will be seen when the gas clears away, and mainly examine the idea that it will be an Algol-type system. More particularly, consideration is given to centrifugally limited accretion as a mechanism to build up a substantial disk, and the presumed evolutionary sequence is from a W Ser to a rapidly rotating Algol to a normal Algol system. Systems such as V367 Cyg and RW Tau fit into this scheme only with difficulty. Because it is extremely difficult to measure the rotation of some W Ser (mass) primaries, it is natural to look at the rotation statistics of Algols to test this idea. The badly behaved light curves and spectroscopy of some Algols (eg. U Cep, RZ Sct) may be attributable to the double contact condition, and the ramifications of this possibility are discussed. If so, the rotation statistics of Algols should show two spikes, corresponding to the two special conditions into which a system should be driven by tidal braking and centrifugally limited spinup. Present rotation statistics do show these spikes. Algols should flip between these states fairly quickly, depending on the mass transfer rate. Thus, to the extent that the meager statistics can be accepted as meaningful, the new (fourth) morphological type of close binary (double contact) has attained demonstrable reality. The rotation statistics are presented in terms of a particular rotation parameter, R, which is zero for synchronism and unity for the centrifugal limit. Future work should develop rotation statistics to see if the rotational lobe-filling (R = 1) spike persists. It should also look into whether W Ser primaries are on the hydrogen burning main sequence, or in general what they are. We also need more light curves of W Ser type systems, high resolution line profiles for the (mass) primaries (with particular attention to the W Ser-Algol transition cases), and spectroscopy of low inclination W Serpentis systems, such as KX And.