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DOE OSTI2022

Autodifferentiable Spectrum Model for High-dispersion Characterization of Exoplanets and Brown Dwarfs

We present an autodifferentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high-dispersion data to fit the ab initio line-by-line spectral computation to the observed spectrum by combining it with the Hamiltonian Monte Carlo in recent probabilistic programming languages. An open-source code, ExoJAX, developed in this study, was written in Python using the GPU/TPU compatible package for automatic differentiation and accelerated linear algebra, JAX. We validated the model by comparing it with existing opacity calculators and a radiative transfer code and found reasonable agreements for the output. As a demonstration, we analyzed the high-dispersion spectrum of a nearby brown dwarf, Luhman 16 A, and found that a model including water, carbon monoxide, and H 2 /He collision-induced absorption was well fitted to the observed spectrum (R = 10 5 and 2.28-2.30 μm). As a result, we found that T0 = 1295$^{-32}_{+35}$ K at 1 bar and C/O = 0.62 ± 0.03, which is slightly higher than the solar value. This work demonstrates the potential of a full Bayesian analysis of brown dwarfs and exoplanets as observed by high-dispersion spectrographs and also directly imaged exoplanets as observed by high-dispersion coronagraphy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

An Extensive Catalog of Early-type Dwarf Galaxies in the Local Universe: Morphology and Environment

We present an extensive catalog of 5405 early-type dwarf (dE) galaxies located in the various environments, i.e., clusters, groups, and fields, of the local universe (z < 0.01). The dEs are selected through visual inspection of the Legacy survey's g–r–z combined tricolor images. The inspected area, covering a total sky area of 7643 deg 2 , encompasses two local clusters (Virgo and Fornax), 265 groups, and the regions around 586 field galaxies of M K < -21 mag. The catalog aims to be one of the most extensive and publicly accessible collections of data on dEs, despite its complex completeness limits, which may not accurately represent its statistical completeness. The strength of the catalog lies in the morphological characteristics, including nucleated, tidal, and ultradiffuse dEs. The two clusters contribute nearly half (2437 out of 5405) of the dEs, and the 265 groups contribute 2103 dEs. There are 864 dEs in 586 fields, i.e., ~1.47 dEs per field. Using a standard definition commonly used in literature, we identify 100 ultradiffuse galaxies (UDGs), which compose ~2% of the dE population. We find that 40% of our sample dEs harbor a central nucleus, and among the UDG population, a majority (79%) are nonnucleated. About 1.3% of dEs suffer from ongoing tidal disturbance by nearby massive galaxies, and only 0.03% show a sign of recent dwarf–dwarf mergers. The association between dEs and their nearest bright neighbor galaxies suggests that dEs are more likely created where their neighbors are non-star-forming ones.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Cooling Delays from Iron Sedimentation and Iron Inner Cores in White Dwarfs

Do white dwarfs have inner cores made of iron? Neutron-rich nuclei like 56 Fe experience a net gravitational force and sediment toward the core. Using new phase diagrams and molecular dynamics simulations, we show that 56 Fe should separate into mesoscopic Fe-rich crystallites due to its large charge relative to the background. At solar abundances, these crystallites rapidly precipitate and form an inner core of order 100 km and 10 -3 M ⊙ that may be detectable with asteroseismology. Associated cooling delays could be up to a Gyr for low-mass white dwarfs but are only ~0.1 Gyr for massive white dwarfs, so while this mechanism may contribute to the Q-branch the heating is insufficient to fully explain it.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs

Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyze atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts' extended pre-main-sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean-analog environments. We contrast the solar system scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and the desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars ≤0.4 M ⊙ experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals, and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favored around FGK stellar types relative to M dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

X-ray emission from white dwarfs.

It is suggested that both the 250-eV background radiation and the recently observed variations in the optical emission from certain white dwarfs may be due to thermal bremsstrahlung from hot coronae surrounding such stars. The X-ray flux from the variable white dwarfs is predicted and in the most favorable case, R548, is sufficiently large to allow an observational test of the model. The possibility that the variations observed in strong X-ray sources are due to pulsations in the coronae surrounding degenerate stars, rather than in stars themselves, is also discussed. For the white dwarfs, both pulsation and rotation of the central star are found to be adequate sources of coronal energy, with pulsation being subject to relatively easy observational test. The energy requirements for the strong X-ray sources are briefly discussed.

Strittmatter, P. A.↗
NASA NTRS1974

Luminosities and temperatures of M dwarf stars from infrared photometry

Bolometric magnitudes for a large number of M type dwarf stars, obtained by broadband infrared photometry at 1.65, 2.2, and 3.5 microns, are reviewed. The data obtained indicate that one parameter is sufficient to describe the blanketing in all of the UBVRI bands for all types of M dwarfs. In general, late M dwarfs seem to have lower effective temperatures than are predicted by theoretical models.

Veeder, G. J.↗
NASA NTRS1976

Skylab ultraviolet stellar spectra - A new white dwarf, HD 149499 B

The letter reports the discovery of a cool star with excess brightness in the vacuum ultraviolet on an objective-prism photograph obtained during the second Skylab mission. This star, HD 149499, is of type K0 V and has a companion with an apparent magnitude of about 11.8; the relatively flat UV spectrum observed at the position of HD 149499 is characteristic of a 10th or 11th magnitude unreddened O- or early B-type star. It is shown that the excess VUV brightness is due to the companion, HD 149499B, which probably lies in the region of the H-R diagram occupied by the hot white dwarfs. Inspection of white dwarf lists indicates that this star is the sixth or seventh brightest white dwarf known. A maximum orbital motion of 0.025 arcsec/yr is estimated along with a period of just under 500 yr.

Parsons, S. B.↗
NASA NTRS1976

White dwarfs, the Galaxy and Dirac's cosmology

The additive and multiplicative versions of Dirac's cosmological hypothesis relating the gravitational constant variation with elapsed time and number of particles populating the universe is invoked to account for the deficiency or absence of white dwarfs fainter than about 0.0001 solar luminosity. An estimate is made of white dwarf luminosity in accordance with the two evolutionary models, and it is conjectured that some old white dwarfs with high space velocities may be on the verge of gravitational collapse. Lack of a special mechanism to produce the vast numbers of black holes or other dead stars accounting for 'missing matter' in the vicinity of the sun and in the galactic halo is noted in Dirac's multiplicative model. Results indicate that either Dirac's theory is untenable, or that radiation and heating are of some unknown nature, or that the process of creation of new matter requires a corresponding input of energy.

Stothers, R.↗
NASA NTRS1977

Feige 7 - A hot, rotating magnetic white dwarf

Results are reported for image-tube-scanner and digicon observations of Feige 7, a faint blue star identified as a probable white dwarf. It is found that this star is a magnetic white dwarf showing a very rich spectrum with Zeeman subcomponents of both hydrogen and neutral helium as well as periodic spectrum and circular-polarization variations. A polarization period of 2.2 hr is computed, and a surface magnetic-field strength of about 18 MG is determined by matching features of the absorption spectrum to Zeeman components. It is suggested that the only reasonable explanation for the periodic variations in circular polarization is an oblique rotator with the spin axis approximately in the plane of the sky and tilted by about 24 deg to the magnetic axis. An effective temperature in the range from 20,000 to 25,000 K is estimated, an absolute magnitude of about 10.5 is derived, and the atmosphere is shown to be helium-dominated. The evolution of Feige 7 is discussed in terms of possible magnetic-field effects on atmospheric composition, rotation velocity (5.5 km/s for a radius of 7000 km), and the origin of white-dwarf magnetic fields.

Liebert, J.↗
NASA NTRS1977

White-dwarf variability and the rotation of g-modes

The multiperiodic behavior clearly evident after Fourier analysis of many DA white-dwarf light curves is interpreted as arising from outbursts whose timing is regulated by the rotation and oscillation of the star. A model is proposed on the basis of three main assumptions: (1) a broad array of g-mode oscillations is taking place all the time in DA white-dwarf variables; (2) slow rotation permits most of the g-modes to couple and form a small group of nonlinear modes, each characterized by one of the indices of the spherical harmonic functions involved; and (3) the observed brightenings of the star are produced by upwelling of convection on a large horizontal scale when the antinodes of the nonlinear oscillation patterns periodically rotate into alignment and cause a temporary local enhancement of energy per unit volume. This model is used to match precisely almost every strong periodicity observed in the complex light curves of the DA white dwarfs G207-9, G38-29, G29-38, and HL Tau 76. If the model has been applied correctly, the results indicate that all four of these variables are rotating slowly with periods in the range from 250 to 500 sec.

Wolff, C. L.↗
NASA NTRS1980

The pulsating white dwarfs

The characteristics of variable white dwarfs are presented through an examination of observational data. The variable white dwarfs are normal, single, DA white dwarfs. The variations are caused by pulsations, but the pulsations are nonradial rather than radial pulsations. The periods are all very long. Excluding harmonics and cross frequencies, the shortest period is 114 sec and the longest period is 1186 sec. Without exception every variable is multiperiodic. The stability of the periods varies from extremely high to very low; the low stability of many of the variables perhaps being due to incomplete data. Finally, there is a strong correlation between the amplitude of the variations and their remaining properties. The higher amplitude variables have more periods in their light curves and the periods are more unstable.

Robinson, E. L.↗
NASA NTRS1979

HEAO-1 observations of dwarf novae

Results are reported for X-ray observations of about 20 dwarf novae in optical outburst with the A-2 experiment aboard HEAO 1. It is shown that only U Gem was detected as an X-ray source during optical outburst and that X-ray emission was observed from SS Cyg, EX Hya, and AY Lyr during optical quiescence. The results obtained indicate that: (1) the soft and hard X-ray emission from U Gem and SS Cyg is linked to the optical state of the system; (2) both U Gem and SS Cyg have one soft and one hard X-ray spectral component; (3) the ratio of soft to hard X-ray energy flux from both U Gem and SS Cyg during outburst is about 10; (4) X-ray emission of the strength and temperature seen from SS Cyg and U Gem is not common among dwarf novae; (5) the hard X-ray flux from SS Cyg and EX Hya probably originates in a hot optically thin plasma; and (6) the variability of the hard X-ray flux from SS Cyg and EX Hya suggests a cooling time scale that would limit the characteristic size of the emission region to a value comparable to the dimensions of a white dwarf.

Mason, K.↗
NASA NTRS1980

The HEAO-A2 soft X-ray survey of dwarf novae in outburst

Although HEAO-1 scanned 19 dwarf novae while they were undergoing optical outbursts, soft X-ray emission (0.18-0.43 keV) was detected only from SS Cygni and U Geminorum. The 3-sigma upper limits to the flux from nondetected objects are presented, which range from 1 to 10 percent of U Gem's mean flux. In addition, hypotheses for the failure to detect soft X-rays from the majority of the dwarf novae are discussed, including interstellar absorption, source variability, and the spottiness of the X-ray coverage. It is concluded that most dwarf novae are not observed as soft X-ray sources by virtue of low temperatures (less than approximately 100,000 K) or greater absorption.

Cordova, F. A.↗
NASA NTRS1981

Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

Nomoto, K.↗
NASA NTRS1981

Observations of strong ultraviolet absorptions in the spectrum of the DC white dwarf LDS 678 B

Evidence from spectroscopic observations that LDS 678 B has strong ultraviolet lines of neutral carbon in its spectrum is presented. The spectra are found to contain strong lines of C I in the ultraviolet and weak He I lines in the visible. The appearance of the star's spectrum and published photometry are consistent with its having an effective temperature near 10,600 K with a helium-dominated atmosphere and a carbon abundance lower than C:He = 0.001. The star's spectra are compared with those of the white dwarfs L879-14 and L145-141, which are cooler than LDS 678B, have C2 bands in the visible and also show strong evidence of strong ultraviolet C I lines. It is concluded that LDS 678 B has a temperature intermediate between the DB and C2 band white dwarfs and that the ultraviolet C I lines can be a useful means for finding carbon in the atmosphere of the cooler white dwarfs.

Wegner, G.↗
NASA NTRS1982

Solidification of carbon-oxygen white dwarfs

The internal structure of white dwarfs is discussed. Highly correlated plasmas are reviewed. Implications for phase separation in the core of cooling white dwarfs are considered. The consequences for evolution of white dwarfs are addressed.

Schatzman, E.↗
NASA NTRS1982

On the coronae of rapidly rotating stars. III - An improved coronal rotation-activity relation in late type dwarfs

While it has previously been claimed that X-ray luminosity, or surface flux, varies as a power of the rotational velocity in G dwarfs, it is demonstrated that a single power law does not provide an acceptable fit to all the data. The data are describable by a power law, with a break in slope near a 12-day period which occurs at the same rotational period as the bifurcation in Vaughan and Preston's (1980) chromospheric study of G dwarfs. It is speculated that the two breaks are both related and due to a change in the stellar dynamo mode, which, at a rotation period of 12 days, corresponds to an age of one billion years for single G dwarfs.

Walter, F. M.↗
NASA NTRS1982

X-ray and UV radiation from accreting nonmagnetic degenerate dwarfs. II

Numerical calculations of X-ray and UV emission from accreting nonmagnetic degenerate dwarfs are reported, which span the entire range of accretion rates and stellar masses. Calculations include the effects of bremsstrahlung, Compton cooling, radiation pressure, albedo of the stellar surface, Compton degradation and free-free abscription of the X-ray spectrum by the accreting matter. Maximum X-ray luminosity for degenerate dwarfs undergoing spherical accretion is found to be 2.2 x 10 to the 36th ergs/s, which is little changed if accretion occurs radially over only a fraction of the stellar surface, so that the emitted radiation escapes without significant scattering. The temperature characterizing the X-ray spectra produced by degenerate dwarfs strongly depends on the stellar mass and the accretion rate, and it is suggested that the correlation between spectral temperature and luminosity is an important signature of degenerate X-ray sources.

Kylafis, N. D.↗