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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

A heating mechanism for the chromospheres of M dwarf stars

The atmospheric structure of the dwarf M-stars which is especially important to the general field of stellar chromospheres and coronae was investigated. The M-dwarf stars constitute a class of objects for which the discrepancy between the predictions of the acoustic wave chromospheric/coronal heating hypothesis and the observations is most vivid. It is assumed that they represent a class of stars where alternative atmospheric heating mechanisms, presumably magnetically related, are most clearly manifested. Ascertainment of the validity of a hypothesis to account for the origin of the chromospheric and transition region line emission in M-dwarf stars is proposed.

Giampapa, M. S.↗
NASA NTRS2019

The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets

Oxygen and methane are considered to be the canonical biosignatures of modern Earth, and the simultaneous detection of these gases in a planetary atmosphere is an especially strong biosignature. However, these gases may be challenging to detect together in the planetary atmospheres because photochemical oxygen radicals destroy methane. Previous work has shown that the photochemical lifetime of methane in oxygenated atmospheres is longer around M dwarfs, but M dwarf planet habitability may be hindered by extreme stellar activity and evolution. Here, we use a 1-D photochemical-climate model to show that K dwarf stars also offer a longer photochemical lifetime of methane in the presence of oxygen compared to G dwarfs. For example, we show that a planet orbiting a K6V star can support about an order of magnitude more methane in its atmosphere compared to an equivalent planet orbiting a G2V star. In the reflected light spectra of worlds orbiting K dwarf stars, strong oxygen and methane features could be observed at visible and near-infrared wavelengths. Because K dwarfs are dimmer than G dwarfs, they offer a better planet-star contrast ratio, enhancing the signal-to-noise (SNR) possible in a given observation. For instance, a 50 hour observation of a planet at 7 pc with a 15-m telescope yields SNR = 9.2 near 1 µm for a planet orbiting a solar-type G2V star, and SNR = 20 for the same planet orbiting a K6V star. In particular, nearby mid-late K dwarfs such as 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be excellent targets for future biosignature searches.

Giada N Arney↗
NASA NTRS2006

A New Spin for Understanding the Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441

In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441 Aims. We investigate the properties of their anomalous horizontal branches (HB) in different photometric planes in order to shed light on the nature of the physical mechanism(s) responsible for the existence of an extended HB blue tail, and of a slope in the HB, visible in all the color-magnitude diagrams. Methods. New photometric data have been collected and carefully reduced. Empirical data have been compared with updated stellar models of low-mass, metal-rich, He-burning structures, transformed to the observational plane with appropriate atmosphere models. Results. We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot Horizontal Branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and w Centauri. We briefly review the theoretical scenarios which have been suggested for interpreting this observational feature. We investigate also on the tilt in the horizontal branch morphology, and provide further evidence that supports early suggestions according to which this feature cannot be interpreted as an effect of differential reddening or radiative levitation, though these effects contribute to create the anomaly. We demonstrate that a possible solution of the puzzle is to assume that a small fraction (approx. 13% in NGC 6388 and approx. 8% NGC 6441) of the stellar population in the two clusters is strongly helium enriched (Y approx. 0.40 in NGC6388 and Y approx. 0.35 in NGC 6441). This solution necessarily implies the presence of a double generation of stars in the two clusters.

Busso, G.↗
DOE OSTI2022

Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N, and TESS

ABSTRACT We present a precise characterization of the TOI-561 planetary system obtained by combining previously published data with TESS and CHEOPS photometry, and a new set of 62 HARPS-N radial velocities (RVs). Our joint analysis confirms the presence of four transiting planets, namely TOI-561 b (P = 0.45 d, R = 1.42 R⊕, M = 2.0 M⊕), c (P = 10.78 d, R = 2.91 R⊕, M = 5.4 M⊕), d (P = 25.7 d, R = 2.82 R⊕, M = 13.2 M⊕), and e (P = 77 d, R = 2.55 R⊕, M = 12.6 R⊕). Moreover, we identify an additional, long-period signal (>450 d) in the RVs, which could be due to either an external planetary companion or to stellar magnetic activity. The precise masses and radii obtained for the four planets allowed us to conduct interior structure and atmospheric escape modelling. TOI-561 b is confirmed to be the lowest density (ρb = 3.8 ± 0.5 g cm−3) ultra-short period (USP) planet known to date, and the low metallicity of the host star makes it consistent with the general bulk density-stellar metallicity trend. According to our interior structure modelling, planet b has basically no gas envelope, and it could host a certain amount of water. In contrast, TOI-561 c, d, and e likely retained an H/He envelope, in addition to a possibly large water layer. The inferred planetary compositions suggest different atmospheric evolutionary paths, with planets b and c having experienced significant gas loss, and planets d and e showing an atmospheric content consistent with the original one. The uniqueness of the USP planet, the presence of the long-period planet TOI-561 e, and the complex architecture make this system an appealing target for follow-up studies.

Lacedelli, G. (ORCID:0000000241977374)↗
NASA NTRS1973

Summary of spacecraft technology, systems reliability, and tracking data acquisition

Goddard activities are reported for 1973. An eight-year flight schedule for projected space missions is presented. Data acquired by spacecraft in the following disciplines are described: stellar ultraviolet, stellar X-rays, stellar gamma rays, solar radiation, radio astronomy, particles/fields, magnetosphere, aurora, and the upper atmosphere.

Source record↗
NASA NTRS1984

Applications of the magneto-optical filter to stellar pulsation measurements

A proposed method of employing the Cacciani magneto-optical filter (MOF) for stellar seismology studies is described. The method relies on the fact that the separation of the filter bandpasses in the MOF can be changed by varying the level of input power to the filter cells. With the use of a simple servosystem the bandpass of a MOF can be tuned to compensate for the changes in the radial velocity of a star introduced by the orbital motion of the Earth. Such a tuned filter can then be used to record intensity fluctuations through the MOF bandpass over an extended period of time for each given star. Also, the use of a two cell version of the MOF makes it possible to alternately chop between the bandpass located in the stellar line wing and a second bandpass located in the stellar continuum. Rapid interchange between the two channels makes it possible for atmospheric-introduced noise to be removed from the time series.

Rhodes, E. J., Jr.↗
NASA NTRS1986

Applications of the magneto-optical filter to stellar pulsation measurements

A proposed method of employing the Cacciani magneto-optical filter (MOF) for stellar seismology studies is described. The method relies on the fact that the separation of the filter bandpasses in the MOF can be changed by varying the level of input power to the filter cells. With the use of a simple servosystem the bandpass of a MOF can be tuned to compensate for the changes in the radial velocity of a star introduced by the orbital motion of the earth. Such a tuned filter can then be used to record intensity fluctuations through the MOF bandpass over an extended period of time for each given star. Also, the use of a two cell version of the MOF makes it possible to alternately chop between the bandpass located in the stellar line wing and a second bandpass located in the stellar continuum. Rapid interchange between the two channels makes it possible for atmospheric-introduced noise to be removed from the time series.

Rhodes, Edward J., Jr.↗
NASA NTRS1988

Fe II emission line profiles in the ultraviolet spectrum of cool, luminous stars

The differences between the outer atmospheres of the coronal and noncoronal cool stars and the importance of massive stellar winds to the latter group are summarized. The utility of Fe II as a probe of such winds is indicated and a brief review of previous observations of Fe II in these stars is given. The early results of a current IUE program to study the chromospheres and winds of the noncoronal, late-type stars are presented, including evidence of a strong dependence of the mid-UV Fe II profiles on stellar luminosity. In addition, the dependence of the Fe II profiles on intrinsic line strength in spectra of Alpha Ori and the discovery of a variation of the Fe II profiles with time in spectra of Gamma Cru are discussed.

Carpenter, Kenneth G.↗
NASA NTRS1989

Models of Pluto's upper atmosphere

Best guesses as to the thermal structure and composition of Pluto's atmosphere have led to speculations of substantial loss rates of methane from the planet over cosmogonic time scales. Results from recent stellar occultation measurements, and using a Parker-type hydrodynamic calculation, show that the loss rates may actually be lower by as much as a factor of about 5, depending upon the efficiency of heating of the atmosphere via the absorption of solar EUV and upon the true atmospheric composition, if the thermal structure of the upper atmosphere is properly taken into account. The loss rate may even be less (by another factor of about 10) if there is minimal heating of the upper atmosphere.

Mcnutt, Ralph L., Jr.↗
NASA NTRS2019

Cool, Evolved Stars: Results, Challenges, and Promises for the Next Decade

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g. Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and to exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade that can provide essential constraints on stellar photospheres, chromospheres, and circumstellar envelopes (CSE), and tie together analyses of the spectra and interferometric and imaging observations of evolved stars. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch (RGB) and red super giant (RSG) stars and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae (PNe) and their relation with the parent Asymptotic Giant Branch (AGB) stars.

Gioia Rau↗
DOE OSTI2023

Cannibals in the thick disk II. Radial-velocity monitoring of the young α -rich stars

Determining ages of stars for reconstructing the history of the Milky Way remains one of the most difficult tasks in astrophysics. This involves knowing when it is possible to relate the stellar mass with its age and when it is not. The young α-rich (YAR) stars present such a case in which we are still not sure about their ages because they are relatively massive, implying young ages, but their abundances are α-enhanced, which implies old ages. We report the results from new observations from a long-term radial-velocity-monitoring campaign complemented with high-resolution spectroscopy, as well as new astrometry and seismology of a sample of 41 red giants from the third version of APOKASC, which includes YAR stars. The aim is to better characterize the YAR stars in terms of binarity, mass, abundance trends, and kinematic properties. The radial velocities of HERMES, APOGEE, and Gaia were combined to determine the binary fraction among YAR stars. In combination with their mass estimate, evolutionary status, chemical composition, and kinematic properties, it allowed us to better constrain the nature of these objects. We found that stars with M < 1 M ⊙ were all single, whereas stars with M > 1 M ⊙ could be either single or binary. This is in agreement with theoretical predictions of population synthesis models. Studying their [C/N], [C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do not follow the APOKASC stars, favoring the scenario that most of them are the product of mass transfer. Our sample further includes two likely undermassive stars, that is to say of such as low mass that they cannot have reached the red clump within the age of the Universe, unless their low mass is the signature of mass loss in previous evolutionary phases. These stars do not show signatures of currently being binaries. Both YAR and undermassive stars might show some anomalous APOGEE abundances for the elements N, Na, P, K, and Cr; although, higher-resolution optical spectroscopy might be needed to confirm these findings. Considering the significant fraction of stars that are formed in pairs and the variety of ways that makes mass transfer possible, the diversity in properties in terms of binarity, and chemistry of the YAR and undermassive stars studied here implies that most of these objects are likely not young.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

We present a search for helium in the upper atmospheres of three sub-Neptune-sized planets to investigate the origins of these ubiquitous objects. The detection of helium for a low-density planet would be strong evidence for the presence of a primary atmosphere accreted from the protoplanetary nebula because large amounts of helium are not expected in the secondary atmospheres of rocky planets. We used Keck+NIRSPEC to obtain high-resolution transit spectroscopy of the planets GJ 1214b, GJ 9827d, and HD 97658b around the 10833 Å He triplet feature. We did not detect helium absorption for any of the planets despite achieving a high level of sensitivity. We used the nondetections to set limits on the planets’ thermosphere temperatures and atmospheric loss rates by comparing grids of 1D models to the data. We also performed coupled interior structure and atmospheric loss calculations, which suggest that the bulk atmospheres (winds) of the planets would be at most modestly enhanced (depleted) in helium relative to their primordial composition. Our lack of detections of the helium triplet for GJ 1214b and GJ 9827d is highly inconsistent with the predictions of models for the present-day mass loss on these planets. Higher signal-to-noise data would be needed to detect the helium feature predicted for HD 97658b. We identify uncertainties in the extreme-ultraviolet fluxes of the host stars and the lack of detailed mass-loss models specifically for cool and metal-enhanced atmospheres as the main limitations to the interpretation of our results. Ultimately, our results suggest that the upper atmospheres of sub-Neptune planets are fundamentally different from those of gas giant planets.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

Spacelab science missions of the 1980's

The objectives and instrumentation of Spacelab missions planned for the 1980s are discussed. Initial flights will be multidisciplinary, serving to verify the performance of Spacelab subsystems and to demonstrate the capability of Spacelab to carry out broad-ranging research in a variety of fields. Further missions will use highly specialized payloads. Among these are the Atmospheres, Magnetospheres and Plasmas (AMPS) Payload, the Astronomy Spacelab Payload (ASP), including solar physics, stellar astronomy and high energy astrophysics variants, and the Spacelab Infrared Telescope Facility. The growing inventory of instruments developed for these missions will then be available for use in different combinations to meet the needs of individual mission objectives proposed by the international scientific community.

Chappell, C. R.↗
NASA NTRS1987

IRAS colors of normal stars

Using stars from the Bright Star Catalog, supplemented by cool dwarf stars from the Gliese catalog, that were detected by IRAS, the authors define empirically the median intrinsic visual-to-infrared color indices for 'normal' stars as a function of IRAS wavelength, spectral type and luminosity class. Anomalously red stars are discussed. Two otherwise undistinguished F giant stars are found with significant excesses at 12 microns. Be stars differ markedly from nonemission B stars in their V-(12) indices due to contamination of the former by free-free emission. Both B and Be stars show large dispersions in V-(25) colors that are associated with the heating of local, but strictly interstellar, dust clouds by some of the non-emission B stars. The derived sequences of stellar colors are closely approximated by either simple blackbody predictions or by model-atmosphere calculations.

Cohen, Martin↗
NASA NTRS1987

VLA observations of rapid 6 cm flux variations in alpha Ori

The red supergiant star alpha Ori was monitored with the Very Large Array (VLA). Thirteen observations at 6 cm show stochastic variations, at the 30 to 40 percent level, with no long term trend. All data was clipped and tapered in AIPS to minimize differences between VLA arrays. The calibration source varied by less than 10 percent over the same interval. The VLA observations of alpha Ori were continued, as well as alpha Her and alpha Sco, at both 2 and 6 cm, to confirm this result and search for long term trends. The stochastic 6 cm flux behavior, with 30 to 40 percent changes on all timescales from the shortest interval of 10 days to the longest, seems at odds with the 400 day periodic variations in U-band photometry and Mg II UV fluxes reported by Dupree, et al. The observed 6 cm flux was 25 percent below the 6 cm flux reported earlier this decade. Several models for the outer atmosphere of alpha Ori place the 6 cm optical depth unity location at several stellar radii above the optical photosphere. The rapid, stochastic variations reported are difficult to reconcile with almost any global process, such as pulsation, Alfven waves or periastron passage.

Bookbinder, J. A.↗
NASA NTRS1992

Goddard Visiting Scientist Program for the Space and Earth Sciences Directorate

A visiting scientist program was conducted in the space and earth sciences at GSFC. Research was performed in the following areas: astronomical observations; broadband x-ray spectral variability; ground-based spectroscopic and photometric studies; Seyfert galaxies; active galactic nuclei (AGN); massive stellar black holes; the differential microwave radiometer (DMR) onboard the cosmic background explorer (COBE); atmospheric models; and airborne and ground based radar observations. The specific research efforts are detailed by tasks.

Kerr, Frank↗
NASA NTRS1993

Solar-Stellar Irradiance Comparison Experiment 1. I - Instrument design and operation

The main objective for the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) is to accurately measure the full disk solar spectral irradiance in the ultraviolet (UV) spectral region over a long time period. To meet this objective, SOLSTICE has the unique capability of making routine observations of the UV radiation from a set of early-type stars, using the identical optical elements and detectors employed for the solar observations. The stars selected for this calibration are assumed, on the basis of stellar evolution theory, to be extremely stable in the UV spectral region. Moreover, it is the average flux from a number of stars, perhaps from as many as 25, that is assumed to be stable. The SOLSTICE 1 is on the Upper Atmosphere Research Satellite (UARS), and the overall design and operation of the instrument are discussed. The quality of the solar and stellar data is extremely high and preliminary results indicate that the technique is working well.

Rottman, Gary J.↗
NASA NTRS1992

Cool stars, stellar systems, and the sun; Proceedings of the 7th Cambridge Workshop, Tucson, AZ, Oct. 9-12, 1991

Consideration is given to HST observations of late-type stars, molecular absorption in the UV spectrum of Alpha Ori, EUV emission from late-type stars, Rosat observations of the Pleiades cluster, a deep ROSAT observation of the Hyades cluster, optical spectroscopy detected by EXOSAT, stellar photospheric convection, a structure of the solar X-ray corona, magnetic surface images of the BY Dra Star HD 82558, a Zebra interpretatin of BY Dra stars, optical flares on II Peg, a low-resolution spectroscopic survey of post-T tauri candidates, millimeter and sub-millimeter emission from flare stars, and activity in tidally interacting binaries. Attention is also given to modeling stellar angular momentum evolution, extended 60-micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars, the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.

Giampapa, Mark S.↗