Jesse L. Greenstein

AN ATLAS OF OPTICAL SPECTRA OF WHITE-DWARF STARS

We present a complete atlas of optical spectra of white dwarf stars. Spectra for all major spectral classes and subclasses are illustrated, and the variation of the dominant spectra features as a function of effective temperature and abundance is documented. We present, as well, spectra of several peculiar or rare types of degenerate stars.

NEUTRAL OXYGEN IN LATE-TYPE STARS.

We have identified the forbidden absorption nebular and auroral lines of oxygen in the spectra of about seventy G and K stars, mostly giants. While weak, they appear in a relatively uncrowded spectral region and can be studied at high dispersion. Coming from the ground state of a dominantly neutral element, the nebular lines are insensitive to differences of model and non-equilibrium effects in line formation. They are formed over a wide range of optical depth and are therefore insensitive to the detailed structure near the boundary temperature. Approximate model atmospheres predict the expected changes in the [O I ] line strength as compared with its strength in the Sun; the models use a T(τ) relation scaled from that in the Sun and opacity tables. The depletion of the free atomic oxygen is caused mostly by CO formation which must be included in detail, We have measured equivalent widths of forbidden oxygen lines in about one-third of the stars and estimated them in the other two-thirds. The measured strengths, from 12 to 140 mA, together with theoretical calculations, lead to the following conclusions: 1. Most stars have a solar abundance ratio of 0/C, near 1 6, and the solar absolute abundances of both these elements. 2. The forbidden lines are observed to be strengthened in the stars with weak CN and high velocity, such as a Boo. These stars are mildly metal deficient (by a factor of 2-4), but have a larger 0/metals ratio than does the Sun. The 0/C ratio and 0/H ratios are probably the same as in the Sun. The observed weakness of CN may be caused primarily by a low N/H ratio. 3. Stars with strong CN such as a Ser have weak [O I ], an 0/C ratio of about 1.2, probably caused by a slight carbon abundance enhancement. The increased CN line strength in these stars is caused primarily by enriched nitrogen. We suggest that this originates during helium flash and mixing 4. Moderately metal-deficient stars, such as HD 148897, have a large 0/metals ratio. It could be that this quite weak-lined star has an oxygen abundance as high as the Sun. 5. The oxygen lines are not observed in the high-velocity, extremely metal-deficient star HD 122563. Oxygen is deficient, but perhaps not as much so as the metals. During the early phases of nucleosynthesis in our Galaxy, the oxygen content increased at a different rate than the metals and reached its present abundance much more rapidly. This may also be true for carbon, but studies of the CN and CH bands and detailed molecular equilibria will be required. Helium burning in massive red giants, or in supermassive stars, apparently proceeded more rapidly than iron-peak element synthesis. Evolution within a star that would lead to increases of surface 0 and C abundances is not probable except in highly evolved stars.

The frequency of hydrogen white dwarfs as observed at high signal to noise ratio

Despite their relatively high stability and slow evolution, white dwarfs are subject to important changes as they cool, since little material is required to contaminate an atmosphere that is only a few meters thick. With an eye to these complexities, about 140 white dwarfs were observed on eight nights in 1982, with about 10,000 to 30,000 photons/pixel being detected near the central wavelengths of the double CCD camera used. Steep Balmer decrements are noted to occur in the cooler degeneration due to increasing He/H ratio. The predominance of non-DA stars as late evolutionary stages prompts questions as to the fate of the accreted and residual H in postasymptotic giant branch evolution. No definitive theoretical argument is found to explain how most of the residual hydrogen is destroyed. 51 references.

Spectrophotometry of white dwarfs as observed at high signal-to-noise ratio. II

CCD spectrophotometry is presented of 140 white dwarfs at high SNR and is analyzed in detail. Energy distributions at 14,000 A are given at bandpasses from 3571 to 8300 A, and equivalent widths of lines of H, He I, metals, and atomic and molecular carbon are given as functions of color for DB, DQ, DZ, and DA stars. New forbidden H I transitions at 6068 A and 6632 A are found in at least the two hottest DB stars, new metallic features are found in cooler DZ stars, and the presence of Ca I in vMa 2 is confirmed. The spectrum of the hot DQAB star G227 - 5 and the pressure-shifted carbon bands seen in 0038-226 are discussed in detail. Comparison of the optical energy distribution of the latter with published IR fluxes shows that the 1-2 micron region is strongly depressed, with extensive blanketing. Equivalent widths, central depths, and width parameters are presented for H-alpha in 73 DA stars in the sample, and their dependences on color are studied. 64 refs.

DEUTERONOMY. SYNTHESIS OF DEUTERONS AND THE LIGHT NUCLEI DURING THE EARLY HISTORY OF THE SOLAR SYSTEM

Abundances in terrestrial and meteoritic matter indicate that the synthesis of deuterons and of the isotopes of lithium, beryllium, and boron occurred during an intermediate stage in the early history of the solar system. In this intermediate stage, the planetary material had become largely separated, but not completely, from the hydrogen which was the main constituent of primitive solar material. Appropriate physical conditions were satisfied by solid planetesimals with dimensions of the order of 10 m and consisting of silicates and oxides of the metals embedded in an icy matrix. The synthesis occurred through spallation and neutron reactions induced in the outer layers of the planetesimals by the bombardment of high-energy charged particles accelerated in magnetic flares at the surface of the condensing sun. The importance of the (n,α) reactions on Li6 and B10 is indicated by the relatively low abundances of these two nuclei. Anomalous abundances of Xe129 and Ag107 observed in meteorites can be attribut...

Spectroscopy of an unusual emission line M star

Moderate-resolution spectroscopy of an unusual late-type faint emission-line star, PC 0025 + 0047, is reported. A very strong (greater than 250 A equivalent width) an H-alpha emission line was detected by the present automated line search algorithm. The spectrum was found to have two unresolved emission lines (H-alpha and H-beta) near zero velocity, superposed on the absorption spectrum of a very red M dwarf which has strong K I, and relatively weak bands of TiO. From the weakness of the subordinate lines of Na I (8192 A) and other spectral features, it is inferred that it is definitely a cooler, and probably fainter, analog of LHS 2924. The strength of the emission lines indicates that PC 0025 + 0447 is very young and may be a fading predecessor brown drawf at an estimated M(bol) approaching 14m at a distance of about 60 pc.

The Faint End of the Main Sequence

New infrared observations of the two faintest known, late M dwarfs, Wolf 359 and +4°4048B (=VB 10) provide accurate luminosities and moderately well-determined temperatures (2500° and 2250° K, respectively). The photometric observations are fitted to a blackbody energy distribution on the assumption that line and band blocking affect most of the spectrum below 1 μ; the temperature structure is taken as that of a gray body. The resulting data, together with Johnsons observations for dM4 and dM5 stars, which have been reanalyzed, calibrate the faint end of the main sequence, with results given in a table and a figure. The bolometric corrections are very large and increase steeply to 6 mag, so that the faintest known stars are, in fact, not very faint; Wolf 359 has L = 13 X 10^(-4) L_☉, and VB 10 has L = 5 X 10^(-4) L_☉. A statistical discussion of Luytens faint red stars of large proper motion gives L = 4 X 10^(-4) L_☉. With a conventional mass-luminosity relation, ℳ ≥ 0.09 ℳ_☉ , for stars of known luminosity. Stars of still lower mass, such as L726-8, are difficult to interpret.

Ultraviolet spectrophotometry of degenerate stars

Observations of one helium- and three hydrogen-atmosphere degenerates made with the International Ultraviolet Explorer are discussed. Fluxes in the UV give temperatures in good accordance with those determined from the ground and from the ANS satellite data. Profiles of the strong L-alpha absorption in two DAs fit predictions for the expected temperatures. Gravity determination is vitiated by their steep temperature dependence. If one accepts that theoretical predictions should be correct, corrections to the absolute IUE calibration derived are an upward shift of 3-5%, with irregular residuals attaining + or - 7%.

An atlas of supernova spectra.

A selection of supernova spectra is given. Most could be calibrated for relative intensities of spectral features. Only a brief attempt is made to interpret these spectra. The atlas should serve as a guide to the observation and interpretation of photographic and spectrophotometric studies of the evolution, for example, of the current bright Type I supernova in NGC 5253. The last section presents some uncalibrated image-tube spectra for the latter. Peculiar types of supernovae are included, with some details of the evolution of spectra of Types I, II, III, and V. Rapid changes and the P Cygni character of lines in Types II, III, and V are displayed. The figures provide data for identification, wavelengths and velocities, and the relative importance of emission and absorption.

FAINT SPECTROPHOTOMETRIC STANDARD STARS FOR LARGE OPTICAL TELESCOPES.I.

The authors present spectral energy distributions over the range λλ3300 - 10000 for a set of five spectrophotometric standards four of which are white dwarfs. These stars are faint (V ≡ 15m.5), equatorial, and exhibit nearly featureless continua, so they can be observed without neutral density filters to calibrate the response of photon-counting spectrographs on large telescopes in both hemispheres.