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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS2022

TOI-3757 b: A Low-density Gas Giant Orbiting a Solar-metallicity M Dwarf

We present the discovery of a new Jovian-sized planet, TOI-3757 b, the lowest-density transiting planet known to orbit an M dwarf (M0V). This planet was discovered around a solar-metallicity M dwarf, using Transiting Exoplanet Survey Satellite photometry and confirmed with precise radial velocities from the Habitable-zone Planet Finder (HPF) and NEID. With a planetary radius of 12.0 (+0.4 -0.5) R ⊕ and mass of 85.3 (+8.8 -8.7)M ⊕ , not only does this object add to the small sample of gas giants (∼10) around M dwarfs, but also its low density (ρ=0.27 +0.05 -0.04 g) provides an opportunity to test theories of planet formation. We present two hypotheses to explain its low density; first, we posit that the low metallicity of its stellar host (∼0.3 dex lower than the median metallicity of M dwarfs hosting gas giants) could have played a role in the delayed formation of a solid core massive enough to initiate runaway accretion. Second, using the eccentricity estimate of 0.14 ± 0.06, we determine it is also plausible for tidal heating to at least partially be responsible for inflating the radius of TOI-3757b b. The low density and large scale height of TOI-3757 b makes it an excellent target for transmission spectroscopy studies of atmospheric escape and composition (transmission spectroscopy measurement of ∼ 190). We use HPF to perform transmission spectroscopy of TOI-3757 b using the helium 10830 Å line. Doing this, we place an upper limit of 6.9% (with 90% confidence) on the maximum depth of the absorption from the metastable transition of He at ∼10830 Å, which can help constraint the atmospheric mass-loss rate in this energy-limited regime.

Exoplanet astronomy↗
NASA NTRS2022

GJ 229B: Solving the Puzzle of the First Known T Dwarf with the APOLLO Retrieval Code

GJ 229B was the first T dwarf to be discovered in 1995, and its spectrum has been more thoroughly observed than that of most other brown dwarfs. Yet a full spectroscopic analysis of its atmosphere has not been done with modern techniques. This spectrum has several peculiar features, and recent dynamical estimates of GJ 229B’s mass and orbit have disagreed widely, both of which warrant closer investigation. With a separation of tens of astronomical units from its host star, GJ 229B falls near the border of the planet and stellar population formation regimes, so its atmosphere could provide clues to formation processes for intermediate objects of this type. In an effort to resolve these questions, we performed retrievals on published spectra of GJ 229B over a wide range of wavelengths (0.5–5.1 μm) using the open-source APOLLO code. Based on these retrievals, we present a more precise mass estimate of 41.6 ± 3.3MJ and an effective temperature estimate of 869 +5 −7 K, which are more consistent with evolutionary models for brown dwarfs and suggest an older age for the system of >1.0 Gyr. We also present retrieved molecular abundances for the atmosphere, including replicating the previously observed high CO abundance, and discuss their implications for the formation and evolution of this object. This retrieval effort will give us insight into how to study other brown dwarfs and directly imaged planets, including those observed with JWST and other next-generation telescopes.

Alex R Howe↗
NASA NTRS2005

Astrometric discovery of GJ 802b : in the Brown Dwarf Oasis?

The Stellar Planet Survey is an ongoing astrometric search for giant planets and brown dwarfs around a sample of ~30 M dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The lowest mass discovery thus far is GJ 802b, a companion to the M5 dwarf GJ 802A. The orbital period is 3.14 +/-0:03 yr, the system mass is 0:214 +/- 0:045 M(circled dot operator), and the semimajor axis is 1:28+/- 0:10 AU or 81 + 6 mas. Imaging observations indicate that GJ 802b is likely to be a brow with the astrometrically determined mass 0:058 +/- 0:021 M(circled dot operator) (1 (sigma) limits). The remaining uncertainty in the orbit is the eccentricity that is now loosely constrained. We dis the system age limits the mass and the prospects of further narrowing the mass range when e is more precisely determined.

stars : low mass↗
NASA NTRS2016

Cloudless Atmospheres for L/T Dwarfs and Extrasolar Giant Planets

The admitted, conventional scenario to explain the complex spectral evolution of brown dwarfs (BDs) since their first detection 20 years ago has always been the key role played by micron-size condensates, called "dust" or "clouds," in their atmosphere. This scenario, however, faces major problems, in particular the J-band brightening and the resurgence of FeH absorption at the L to T transition, and a physical first-principle understanding of this transition is lacking. In this Letter, we propose a new, completely different explanation for BD and extrasolar giant planet (EGP) spectral evolution, without the need to invoke clouds. We show that, due to the slowness of the CO/ CH4 and N2/NH3 chemical reactions, brown dwarf (L and T, respectively) and EGP atmospheres are subject to a thermo-chemical instability similar in nature to the fingering or chemical convective instability present in Earth oceans and at the Earth core/mantle boundary. The induced small-scale turbulent energy transport reduces the temperature gradient in the atmosphere, explaining the observed increase in near-infrared J-H and J-K colors of L dwarfs and hot EGPs, while a warming up of the deep atmosphere along the L to T transition, as the CO/CH4 instability vanishes, naturally solves the two aforementioned puzzles, and provides a physical explanation of the L to T transition. This new picture leads to a drastic revision of our understanding of BD and EGP atmospheres and their evolution.

Brown dwarf stars↗
NASA NTRS2018

Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

Time-resolved observations of brown dwarfs' rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 (WFC3) near-infrared G141 taken in six consecutive orbits observations of HNPeg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the 1.1 to 1.7 micron broadband light curve has the amplitude of and period of hour. The modulation amplitude has no detectable wavelength dependence except in the 1.4 micron water absorption band, indicating that the characteristic condensate particle sizes are large (greater than 1 micron). We detect significantly (4.4 sigma) lower modulation amplitude in the 1.4 micron water absorption band, and find that HN Peg B's spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.

brown dwarf↗
NASA NTRSDate not supplied

JWST Time Series Spectra of a Very Rapidly Rotating T-Dwarf

The T7 brown dwarf 2MASS J03480772−6022270 is an extremely rapidly rotating object with a rotation period of only 1.1 hours, the most rapid brown dwarf rotator known to date (Tannock, et al. 2021). This rotation period is close to the dynamical instability limit, providing an extremum test case for the effects of rotation in a brown dwarf atmosphere. This object was observed with the JWST NIRSpec and MIRI instruments as part of the GTO Program 1189 in August, 2022, and included observations over two full rotation periods using the NIRSpec prism mode, as well as higher-resolution spectra over the both the NIRSpec and MIRI wavelength ranges. The sensitivity of the JWST instruments yielded excellent signal-to-noise spectra that will be presented here that deviate significantly from the model predictions.

astronomy↗
NASA NTRS2023

JWST Observations of the Enigmatic Y Dwarf WISE 1828+2650: I. Limits to a Binary Companion

The Y-dwarf WISE 1828+2650 is one of the coldest known Brown Dwarfs with an effective temperature of ~300 K. Located at a distance of just 10 pc, previous model-based estimates suggest WISE 1828+2650 has a mass of ~5-10 M J , making it a valuable laboratory for understanding the formation, evolution and physical characteristics of gas giant planets. However, previous photometry and spectroscopy have presented a puzzle with the near-impossibility of simultaneously fitting both the short (0.9-2.0 μm) and long wavelength (3-5 μm) data. A potential solution to this problem has been the suggestion that WISE 1828+2650 is a binary system whose composite spectrum might provide a better match to the data. Alternatively, new models being developed to fit JWST/NIRSpec and MIRI spectroscopy might provide new insights. This article describes JWST/NIRCam observations of WISE 1828+2650 in 6 filters to address the binarity question and to provide new photometry to be used in model fitting. We also report Adaptive Optics imaging with the Keck 10 m telescope. We find no evidence for multiplicity for a companion beyond 0.5 AU with either JWST or Keck. Companion articles will present low and high resolution spectra of WISE 1828 obtained with both NIRSpec and MIRI.

Infrared spectroscopy↗
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.↗