Low-mass Main-sequence Stars Research Articles

Asteroseismic determination of helium abundance in stellar envelopes

ABSTRACT Intermediate degree modes of the solar oscillations have previously been used to de-termine the solar helium abundance to a high degree of precision. However, we cannotexpect to observe such modes in other stars. In this work we investigate whether lowdegree modes that should be available from space-based asteroseismology missionscan be used to determine the helium abundance, Y, in stellar envelopes with sufficientprecision. We find that the oscillatory signal in the frequencies caused by the depres-sion in Γ 1 in the second helium ionisation zone can be used to determine the envelopehelium abundance of low mass main sequence stars. For frequency errors of 1 part in10 4 , we expect errors σ Y in the estimated helium abundance to range from 0.03 for0.8M ⊙ stars to 0.01 for 1.2M ⊙ stars. The task is more complicated in evolved stars,such as subgiants, but is still feasible if the relative errors in the frequencies are lessthan 10 −4 .Key words: Stars: oscillations; Stars: abundances

The effect of star-spots on eclipse timings of binary stars

We investigate the effects that star-spots have on the light curves of eclipsing binaries, and in particular how they may affect the accurate measurement of eclipse timings. Concentrating on systems containing a low-mass main-sequence star and a white dwarf, we find that if star-spots exhibit the Wilson depression they can alter the times of primary eclipse ingress and egress by several seconds for typical binary parameters and star-spot depressions. In addition, we find that the effect on the eclipse ingress/egress times becomes more profound for lower orbital inclinations. We show how it is possible, in principle, to determine estimates of both the binary inclination and the depth of the Wilson depression from light curve analysis The effect of depressed star-spots on the O−C diagrams of eclipsing systems is also investigated. It is found that the presence of star-spots will introduce a jitter in the O−C residuals and can cause spurious orbital period changes to be observed. Despite this, we show that the period can still be accurately determined even for heavily spotted systems.

Rotational Evolution of Intermediate and Low Mass Main Sequence Stars

Bob Kraft (1967) showed that there is a break in the mean rotational velocity of stars at about spectral type F5, with more massive stars generally being rapid rotators and less massive stars generally being slow rotators. He also showed that in the late F spectral range at least, there is an evolution with time on the main sequence, with younger F stars being more rapidly rotating. Kraft's observational database extended only to about one solar mass due to the sensitivy limitations of photographic plates. Modern observations of low mass stars in open clusters, extending down in mass to nearly the hydrogen burning mass limit in a few clusters, have since been used to show that rotational spindown is the common feature of stars less massive than the sun but that there is a wide spread in rotational velocities when stars arrive on the ZAMS. I will review what is known empirically concerning the rotational velocities of intermediate and low mass field stars, using the open cluster data to place the field star observations in context.

Modeling the Accretion Stream in Polars

I present a summary of my dissertation research into a method of modeling the accretion stream in magnetic cataclysmic variables known as polars. Polars are interacting binary stars in which the primary star is a magnetic white dwarf with a surface field strength 10 MG. Material is accreted from a low-mass main-sequence secondary star and funneled by the magnetic field onto the white dwarf surface. The standard, simple picture of the accretion stream has two regions: the ballistic path where the magnetic forces are negligible and the magnetically controlled path where the material is forced to flow along the magnetic field lines onto the white dwarf surface. The “coupling” point is located where the gas stream transitions from the ballistic path to the magnetically controlled path. The stream is assumed to be a thin tube with a cylindrically symmetric density distribution. This simple picture, however, is unable to explain the complex features often seen in polar observations. In order to develop a better understanding of the complex accretion streams, a more detailed model is needed. My models solve for the three-dimensional stream geometry and provide the density distribution and gas velocity throughout the stream from the L1 point to the white dwarf surface. In order to solve for the detailed stream structure without resorting to simplified geometries, we must employ computationally intensive algorithms such as smoothed particle hydrodynamics (SPH). SPH treats a continuous gas as a superposition of extended segments of the gas, referred to as particles. The hydrodynamic equations can then be approximated by summations over the particles. In my SPH models, the gas particles are released from the L1 boundary and then allowed to flow under the influence of the gravitational, rotational, and thermodynamic forces. These initial models use the common assumption that the magnetic pressure is negligible as long as the ram pressure of the gas is higher. Once the magnetic pressure on a particle exceeds the ram pressure, the particle “couples” and is forced to flow along the magnetic field lines. The SPH method has been applied to a “reference system” that has system parameters typical of what is found in known polars. The reference system results are in general agreement with the standard, simplified model of the accretion stream but show considerably more detail. Interesting features include the coupling region and the impact region on the white dwarf surface. The coupling region is significantly extended, approximately 11 white dwarf radii long. The low-density edges of the stream couple to the field lines first, while the higher density core of the stream is able to penetrate farther into the magnetosphere of the white dwarf. In addition, because of the orientation of the magnetic dipole axis to the stream, the edge of the stream closest to the dipole will begin to couple before the far edge. The resulting shape is an asymmetric cone. The basic structure in the coupling region is projected down to the white dwarf surface at the impact region. Material from the stream impacts the surface in an arc shape offset from the dipole axis. One end of the arc, corresponding to the material that coupled from the core of the stream, has a much higher density than material impacting along the remainder of the arc. The arc is more accurately described as a V shape with two wings extending out from the high-density core region. The impacting density drops off along these wings, which are also asymmetric. In addition to the stream geometry described above, the reference SPH model reveals another feature not seen in the standard simple accretion-stream model. While the vast majority of the gas in the stream flows away from the L1 point along the ballistic stream, a small fraction of the material is predicted to be controlled by the magnetic field immediately and couple to the lines directly at the L1 point. This small amount of material would flow along a distinct path and impact the white dwarf away from the main arc-shaped impact region. The initial reference model work was supplemented with additional SPH models that explore the effects of various system parameters on the accretion stream. In particular, the magnetic field strength, mass-transfer rate, dipole orientation, and stream temperature were varied. As expected, the coupling position depends upon the mass-transfer rate and magnetic field strength. The stream temperature, which is assumed to be isothermal, does not significantly affect the coupling position but does affect the width of the accretion stream. The dipole orientation naturally affects the geometry of the magnetically confined path and impact location on the white dwarf but shows

Canonical Extra Mixing in Low‐Mass Red Giants

We have used the latest observational data on the evolutionary variations of the surface chemical composition in low-mass metal-poor stars, both in the field and in globular clusters, to constrain the basic properties of extra mixing in upper red giant branch (RGB) stars. Two different models of extra mixing have been incorporated into a stellar evolution code: a parametrical diffusion model and a model with rotation-induced turbulent diffusion. Application of the first model to the interpretation of the observed variations of the surface abundances of Li, C, and N and of the isotopic ratio 12C/13C in field stars has revealed that, for the majority of upper RGB stars, the depth and rate of extra mixing do not appear to vary appreciably from star to star. Furthermore, comparisons of our calculations with the results obtained by other authors show that at least the extra mixing depth does not seem to depend strongly on metallicity. Therefore, we propose to call this universal nonconvective mixing process canonical extra mixing. We also put forward the hypothesis that some of the upper RGB stars may be experiencing enhanced extra mixing, which is much faster (by a factor of ~100) and somewhat deeper than canonical extra mixing. This could explain the phenomenon of Li-rich giants. Enhanced extra mixing could also contribute to the O-Na and Mg-Al anticorrelations that are seen in some globular cluster red giants. A possible mechanism of extra mixing in upper RGB stars may be turbulent diffusion or/and meridional circulation induced by rotation. In this case, enhanced extra mixing requires rotational velocities that are ≈10 times as fast as those that are sufficient for the occurrence of canonical extra mixing. Observations do not exclude this possibility because (1) the dispersion in the surface rotational velocities of field Li-rich giants span a range of a factor of ~10 and (2) the extremely fast rotation of blue horizontal branch stars in globular clusters may require that their RGB precursors had been spun up appreciably by an external source. Star-to-star abundance variations in globular clusters may well have been produced as the result of both evolutionary and primordial processes. In the primordial scenario, the nuclearly processed material that is accreted by low-mass main-sequence stars may have originated primarily in earlier generations of massive asymptotic giant branch stars that had undergone hot bottom burning of their envelopes and partly in mass-losing upper RGB stars that had been just a bit more massive than the present-day main-sequence turnoff stars and had experienced extra mixing in the past.

Modelling collision products of triple-star mergers

In dense stellar clusters, binary–single and binary–binary encounters can ultimately lead to collisions involving two or more stars during a resonant interaction. A comprehensive survey of multistar collisions would need to explore an enormous amount of parameter space, but here we focus on a number of representative cases involving low-mass (0.4-, 0.6- and 0.8-M⊙) main-sequence stars. Using both smoothed particle hydrodynamics (SPH) calculations and a much faster fluid sorting software package (mmas), we study scenarios in which a newly formed product from an initial collision collides with a third parent star. By varying the order in which the parent stars collide, as well as the orbital parameters of the collision trajectories, we investigate how factors such as shock heating affect the chemical composition and structure profiles of the collision product. Our simulations and models indicate that the distribution of most chemical elements within the final product is not significantly affected by the order in which the stars collide, the direction of approach of the third parent star, or the periastron separations of the collisions. Although the exact surface abundances of beryllium and lithium in the product do depend on the details of the dynamics, these elements are always severely depleted due to mass loss during the collisions. We find that the sizes of the products, and hence their collisional cross-sections for subsequent encounters, can be sensitive to the order and geometry of the collisions. For the cases that we consider, the radius of the product formed in the first (single–single star) collision ranges anywhere from roughly 2–30 times the sum of the radii of its parent stars. The size of the final product formed in our triple-star collisions is more difficult to determine, but it can easily be as large or larger than a typical red giant. Although the vast majority of the volume in such a product contains diffuse gas that could be readily stripped in subsequent interactions, we nevertheless expect the collisional cross-section of a newly formed product to be greatly enhanced over that of a thermally relaxed star of the same mass. Our results also help establish that the algorithms of mmas can quickly reproduce the important features of our SPH models for these collisions, even when one of the parent stars is itself a former product.

Angular momentum transport by internal gravity waves

We examine the generation of gravity waves by the surface convection zone of low-mass main sequence stars with solar metallicity. It is found that the total momentum luminosity in waves rises with stellar mass, up to the quasi-disappearance of the convection zone around 6500K (corresponding to a mass of about 1.4 Msun for solar metallicity) where the luminosity drastically drops. We calculate the net momentum extraction associated with these waves and explain how the calculated mass dependence helps resolve the enigma of the Li dip in terms of rotational mixing, forming a coherent picture of mixing in all main sequence stars.

A period investigation of two chromospherically active binary stars: RT Coronae Borealis and PW Herculis

Orbital period variations of two chromospherically active binary systems, RT CrB and PW Her, are presented. It is shown that the orbital period of RT CrB undergoes a cyclic oscillation with a period of 53.9 years. For PW Her, an alternate change, with a period of 42.7 years, is found to superimpose on a rapid secular increase (d P/d t=+3.53×10 −6 days/year). If the period oscillations of those two systems are caused by the light-time effect of a third body, the analysis for RT CrB indicates that the third body would be a low-mass main-sequence star, while, for PW Her, the mass of the third body should be no less than 7.8 M ⊙. Since no spectral lines of the third body were seen in PW Her from the spectroscopic study by Popper [AJ 100 (1990) 247], if there is a third body in the system, it can only be a black hole. However, as both components in the two binary stars were showing strong chromospheric activity, the alternate period variations are more plausibly explained as the result of magnetic activity cycles. No secular period changes of RT CrB are found, which is in agreement with the detached evolved configuration of the system. The long-term period increase of PW Her may indicate that it is on an active phase of mass transfer (d m/d t=2.17×10 −6 M ⊙/year).

Eclipsing binary millisecond pulsar PSR J1740-5340 – evolutionary considerations and observational test

We perform evolutionary calculations for a binary system to produce observed binary parameters for the PSR J1740-5340. Our calculations support the model proposed by D'Amico et al. (2001) in which this binary may be the progenitor of a millisecond pulsar + helium white dwarf system. We propose an observational test to verify this hypothesis. We propose that observations of abundances of C, N, O elements in the spectra of the optical companion of PSR J 1740-5340 may give additional information about the nature of the secondary being either i) a perturbed low-mass main-sequence star or ii) an evolved star with helium core and thin hydrogen envelope which has lost its main envelope during semi-detached evolution.

Gravitational Waves from Stars Orbiting the Sagittarius A* Black Hole

One of the main astrophysical processes leading to strong emission of gravitational waves to be detected by the future space-borne interferometer Laser Interferometer Space Antenna (LISA) is the capture of a compact star by a black hole with a mass of a few million solar masses in the center of a galaxy. In previous studies, main-sequence stars were thought not to contribute because they suffer from early tidal disruption. Here we show that according to our simulations of the stellar dynamics of the Sgr A* cluster, there must be one to a few low-mass main-sequence stars sufficiently bound to the central Galactic black hole to be conspicuous sources in LISA observations. The probability that a white dwarf may be detectable is lower than 0.5, and in spite of mass segregation, detection of a captured neutron star or stellar black hole in the center of the Milky Way is highly unlikely.

The Giant X‐Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?

We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 as the tidal disruption of a star by a supermassive black hole. From the observed rapid decline of the luminosity, over a timescale of a few years, we argue that the flare was powered by the fallback of debris rather than subsequent accretion via a thin disk. The fallback model allows constraints to be set on the black hole mass and the mass of debris. The latter must be very much less than a solar mass to explain the very low luminosity of the flare. The observations can be explained either as the partial stripping of the outer layers of a low-mass main sequence star or as the disruption of a brown dwarf or a giant planet. We find that the X-ray emission in the flare must have originated within a small patch rather than over the entire torus of circularized material surrounding the black hole. We suggest that the patch corresponds to the ``bright spot'' where the stream of returning debris impacts the torus. Interestingly, although the peak luminosity of the flare was highly sub-Eddington, the peak flux from the bright spot was close to the Eddington limit. We speculate on the implications of this result for observations of other flare events.

Binaries for LISA

Binaries sufficiently compact to be observed with LISA consist of smallstars, with both chosen from the followinglist: low-mass main sequence stars, whitedwarfs, neutron stars, black holes orhelium-burning cores of high-mass stars.The numbers of various types of binariescan be derived from observations, providedthat observational selection effects aretaken into account. They can also bederived from theoretical populationsynthesis, but the uncertainties are suchthat the resulting numbers must be gaugedwith observed samples.

The 'second parameter': a memory from the globular cluster formation epoch

We study the correlations between the distribution of stars on the horizontal branch (HB), the HB morphology, and some other properties of globular clusters (GCs) in a large sample of GCs. We strengthen previous results that a general correlation exists only between HB morphology and metallicity. Correlations with other properties, e.g. central density and core radius, exist only for GCs within a narrow metallicity range. We conjecture that the lack of correlations with present properties of GCs (besides metallicity) is because the variation of the HB morphologies between GCs having similar metallicities is caused by a process, or processes, the effect of which was determined at the formation time of GCs. This process (or processes) is historically termed the ‘second parameter’, metallicity being the ‘first parameter’. We then argue that the ‘planet second parameter’ model, where the presence of planets and to a lesser degree brown dwarfs and low-mass main-sequence stars is the ‘second parameter’, fits this conjecture. This is because the processes that determine the presence of planets and their properties occur during the formation epoch of the star and its circumstellar disc.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] NICMOS Images of Herbig-Haro Energy Sources: [[CLC]Fe[/CLC] [CSC]ii[/CSC]] Jets, Binarity, and Envelope Cavities

We have observed seven regions surrounding the driving sources of Herbig-Haro flows using the NICMOS infrared camera on board the Hubble Space Telescope. These Herbig-Haro energy sources, which power the HH 34, 47, 83, 111, 199, 300, and 454 flows, have all been previously detected in the centimeter-wavelength radio continuum. The regions were imaged in two broadband filters, F160W and F205W, which, in addition to stellar continuum, principally transmit the [Fe II] 1.644 μm and H2 2.122 μm lines, respectively, of shocks present in the images. Highly collimated infrared jets emerge from the sources of HH 34, 111, 300, and 454. In all of these cases, the [Fe II] emission transmitted in the F160W filter is much stronger than the H2 emission in the F205W band. Combined with previously published NICMOS images of the bright [Fe II] HH 1 jet, these data suggest that [Fe II] emission may be as important a tracer of shocked jets in the infrared as [S II] emission is in the optical. [Fe II] emission may be enhanced near the driving sources relative to the H2 emission by a combination of high jet density and strong far-ultraviolet (7.6–13.6 eV) radiation from the forming star. The HH 454 [Fe II] jet, which emanates from the L1551 NE source, is pointed directly at the bright shock HH 29, providing further evidence that this young star, rather than L1551 IRS 5, is the source of the brightest Herbig-Haro object in the L1551 outflow complex. In five regions, the driving sources are visible at 2 μm. Among these, the sources powering HH 47 and HH 300 are found to be binary stars. One other young stellar system, the source of the HH 111 protostellar jet, is triple. When combined with other studies, these results indicate that 36% of 14 Herbig-Haro energy sources observed with NICMOS are multiple at near-infrared wavelengths with component separations greater than 015–020. Therefore, considering incompleteness, obscuration, and faintness of possible companions in the observed bands, the binary frequency is likely to be considerably higher among the selected systems than among low-mass main-sequence dwarf stars. There is no obvious connection between source multiplicity and jet morphology observed on large scales. For example, the beam of the HH 47 jet exhibits quasi-periodic wiggles that might be one indication of jet modulation at the binary orbital period. However, the total mass of the observed central binary would have to be unrealistically large to make the orbital period comparable to the ejection time interval of these jet features. Finally, the properties of the near-infrared reflection nebulae associated with the observed sources are investigated. The shapes of the cavity walls traced by the observed reflection nebulae indicate the presence of large-scale and highly flattened circumstellar structures that are opaque at near-infrared wavelengths. Their flaring shapes either are the result of preferential infall of the protostellar envelope along the outflow axis or were carved by wide-angle winds emerging from the inner disk and the forming star. The inclination angles of several circumstellar structures are estimated.

The SOHO–Stellar Connection

The ‘solar– stellar connection’ bridges the daytime and nighttime communities; an essential link between the singular, but detailed, views of our Sun, and the broad, but coarse, glimpses of the distant stars. One area in particular – magnetic activity – has profited greatly from the two way traffic in ideas. In that spirit, I present an evolutionary context for coronal activity, focusing on the very different circumstances of low-mass main-sequence stars like the Sun, compared with more massive stars. The former are active mainly very early in their lives, whereas the latter become coronal only near the end of theirs, during the brief incursion into the cool half of the Hertzsprung–Russell diagram as yellow, then red, giants. I describe tools at the disposal of the stellar astronomer; especially spectroscopy in the ultraviolet and X-ray bands where coronae leave their most obvious imprints. I compare HST STIS spectra of solar-type dwarfs – ζ Dor (F7 V), an active coronal source, and α Cen A (G2 V), near twin of the Sun – to the SOHO SUMER UV solar atlas. I also compare the STIS line profiles of the active coronal dwarf to the corresponding features in the mixed activity ‘hybrid chromosphere’ bright giant α TrA (K2 II) and the archetype ‘non-coronal’ red giant Arcturus (α Boo; K2 III). The latter shows dramatic evidence for a ‘cool absorber’ in its outer atmosphere that is extinguishing the ‘hot lines’ (like Si iv λ1393 and N v λ1238) below about 1500 A; the corona of the red giant seems to lie beneath its extended chromosphere, rather than outside as in the Sun. I present an early taste of the moderate resolution spectra we can expect from the recently launched Chandra X-ray Observatory (CXO), and contemporaneous STIS high resolution UV measurements of the CXO calibration star Capella (α Aur; G8 III + G1 III). Last, I describe preliminary results from a May 1999 observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer (IRIS). The purpose was to explore the dynamics of the quiet solar atmosphere through the key ‘magnetic transition zone’ that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar phenomena to properties of the average line shapes (widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying physical insights from the solar setting to the realm of the distant stars.

On the Formation of Helium Double Degenerate Stars and Pre–Cataclysmic Variables

The evolution of low-mass (M 2.5 binaries through the common envelope phase has been M _ ) studied for systems in which one member is on its —rst ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1¨2 with degen- M _ ) erate helium cores (0.28¨0.45 and companions (0.1¨0.45 for initial orbital periods ranging from M _ ) M _ ) D15 to 1000 days. The results suggest that these low-mass binary systems can survive the common envelope phase provided that the helium degenerate core is more massive than about 0.2¨0.25 and M _ that the mass of the red giant progenitor is Speci—c applications are made to observed double (2 M _ . helium degenerate systems, precataclysmic variables, and subdwarf B stars in order to place constraints on progenitor systems evolving through the common envelope phase. For the observed short-period double degenerate systems, it is found that evolutionary scenarios involving two phases of common envelope evolution are not likely and that a scenario involving an Algol-like phase of mass transfer fol- lowed by a common envelope phase is viable, suggesting that the —rst-formed white dwarf is often reheated by nuclear burning on its surface. A formation mechanism for two subdwarf B stars observed in eclipsing short-period binaries with low-mass main-sequence stars is also described. Subject headings: binaries: closecircumstellar matterhydrodynamics ¨ novae, cataclysmic variablesstars: evolutionstars: interiors

Rotation, planets, and the 'second parameter' of the horizontal branch

We analyze the angular momentum evolution from the red giant branch (RGB) to the horizontal branch (HB) and along the HB. Using rotation velocities for stars in the globular cluster M13, we find that the required angular momentum for the fast rotators is up to 1-3 orders of magnitude (depending on some assumptions) larger than that of the sun. Planets of masses up to five times Jupiter's mass and up to an initial orbital separation of about 2 AU are sufficient to spin-up the RGB progenitors of most of these fast rotators. Other stars have been spun-up by brown dwarfs or low-mass main sequence stars. Our results show that the fast rotating HB stars must have been spun-up by companions while they evolved on the RGB. The angular momentum considerations presented in this paper further supports the planet parameter model. In this model the second parameter process is interaction with low-mass companions, in most cases gas giant planets, and in a minority of cases with brown dwarfs or low-mass main sequence stars.

Sleuthing the Dynamo. II.Hubble Space TelescopeGoddard High Resolution Spectrograph Observations of Solar‐Type Dwarfs in Young Galactic Clusters

Far-ultraviolet (1150-1670 A) spectra of three solar-type dwarfs in the young galactic clusters α Persei and the Pleiades were obtained with the Hubble Space Telescope Goddard High-Resolution Spectrograph (G140L mode). Time series of Si IV λ1393 show erratic variability on timescales of tens of minutes. An X-ray/C IV flux-flux diagram displays a power-law slope of ~2, up to the level of the most active cluster stars. These fall systematically below the extrapolation owing to X-ray saturation. In rotation-activity diagrams, the X-rays plateau above ~15 km s-1 (P ~ 3 days), while C IV continues to rise until ~35-50 km s-1 (P ~ 1-1.5 days). Such behavior is significant for understanding the magnetic dynamo at high rotational velocities and is relevant for predicting the ionizing environment bathing newly formed planets around low-mass main-sequence stars.

A Wide Symbiotic Channel to Type Ia Supernovae

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low mass red-giant (RG), where strong winds from the accreting WD play a key role to increase the WD mass to the Chandrasekhar mass limit. Here we propose two new evolutionary processes which make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD + RG close binary can form from a wide binary even with such a large initial separation as $a_i \lesssim 40000 R_\odot$. Such a binary consists of an AGB star and a low mass main-sequence (MS) star, where the AGB star is undergoing superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Therefore, the WD + RG binary can form from much wider binaries than our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds 1.15. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy.

Observation of the Halo of the Edge-On Galaxy IC 5249

Optical photometry and H I synthesis observations of the southern edge-on Sc/Sd galaxy IC 5249 are reported. The rotation curve rises linearly out to a radius of 7 kpc and then appears to flatten out at ~100 km s-1. The H I mass out to 24.5 kpc is ~6 × 109 M⊙, or 10% of the total mass out to this radius. The color, central surface brightness, scale height, and scale length of the disk of IC 5249 are R - I ≈ 0.4, μ = 20.6 ± 0.1RC mag arcsec-2, 600 ± 40 pc, and 11 ± 2 kpc, respectively. Additional light to that predicted by an exponential disk is present at distances greater than 3 kpc from the disk. At 5 kpc the surface brightness is 27–28RC mag arcsec-2. The measured distribution of surface brightness is used to constrain the abundance of low-mass main-sequence stars in the halo of the galaxy. A halo made up entirely of main-sequence stars heavier than 0.13 M⊙ is excluded. We also find that less than 20% of the halo can be composed of main-sequence stars heavier than 0.30 M⊙. Further observations are required to determine the rotation curve of IC 5249 to large radii and to determine precisely the abundance of low-mass main-sequence stars in the halo of the galaxy.