Search NASA⌕ Search

SEARCH · Search NASA

Results for “MAIN SEQUENCE STAR”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Intrinsic ultraviolet colors from OAO-2 Celescope observations for stars on the main sequence.

Intrinsic ultraviolet colors (Vi-V)o have been determined with ultraviolet magnitudes derived from several hundred observations taken with the photometers of OAO-2's Celescope experiment. Relationships between intrinsic ultraviolet color, (B-V)o, and spectral class are presented for main-sequence stars from B 0 to G 3. A brief description of the Celescope and its calibration is included, and a comparison of the determined colors is made with theoretical colors.

Haramundanis, K.↗
NASA NTRS1994

Close encounters of the third-body kind

We simulated encounters involving binaries of two eccentricities: e = 0 (i.e., circular binaries) and e = 0.5. In both cases the binary contained a point mass of 1.4 solar masses (i.e., a neutron star) and a 0.8 solar masses main-sequence star modeled as a polytrope. The semimajor axes of both binaries were set to 60 solar radii (0.28 AU). We considered intruders of three masses: 1.4 solar masses (a neutron star), 0.8 solar masses (a main-sequence star or a higher mass white dwarf), and 0.64 solar masses (a more typical mass white dwarf). Our strategy was to perform a large number (40,000) of encounters using a three-body code, then to rerun a small number of cases with a three-dimensional smoothed particle hydrodynamics (SPH) code to determine the importance of hydrodynamical effects. Using the results of the three-body runs, we computed the exchange across sections, sigma(sub ex). From the results of the SPH runs, we computed the cross sections for clean exchange, denoted by sigma(sub cx); the formation of a triple system, denoted by sigma(sub trp); and the formation of a merged binary with an object formed from the merger of two of the stars left in orbit around the third star, denoted by sigma(sub mb). For encounters between either binary and a 1.4 solar masses neutron star, sigma(sub cx) approx. 0.7 sigma(sub ex) and sigma(sub mb) + sigma(sub trp) approx. 0.3 sigma(sub ex). For encounters between either binary and the 0.8 solar masses main-sequence star, sigma(sub cx) approx. 0.50 sigma(sub ex) and sigma(sub mb) + sigma(sub trp) approx. 1.0 sigma(sub ex). If the main sequence star is replaced by a main-sequence star of the same mass, we have sigma(sub cx) approx. 0.5 sigma(sub ex) and sigma(sub mb) + sigma(sub trp) approx. 1.6 sigma(sub ex). Although the exchange cross section is a sensitive function of intruder mass, we see that the cross section to produce merged binaries is roughly independent of intruder mass. The merged binaries produced have semi-major axes much larger than either those of the original binaries or those of binaries produced in clean exchanges. Coupled with their lower kick velocities, received from the encounters, their larger size will enhance their cross section, shortening the waiting time to a subsequent encounter with another single star.

Davies, M. B.↗
NASA NTRS1993

A survey for rapid variability among early main-sequence A stars

A survey of nonpeculiar early (A0-A5) main-sequence A stars for rapid, low-amplitude variability is presented. The survey is also an independent test of the main-sequence mass-loss theory proposed by Willson et al. (1986). All observations were gathered with the University of Wisconsin Two-Star Photometer. Several period-searching methods are used to analyze time series of differential-photometric data. This instrument coupled with a computerized high-speed data collection system was used with small telescopes at Pine Bluff Observatory and Table Mountain Observatory. Several period-search methods are used to analyze time series of differential-photometric data. This instrument and data-reduction technique allow millimagnitude light variations to be detected. The survey also discovered several low-amplitude Delta Sct stars, all of which are in or blueward of the recognized instability strip.

Schutt, Randy L.↗
NASA NTRS1973

The ultraviolet flux envelopes of main-sequence B stars.

Flux envelopes in the range from 1100 to 6000 A for representative, nearby, thus unreddened main sequence stars are assembled. These envelopes are compared with the flux envelopes predicted from the classical LTE theory of model atmospheres. An investigation is conducted concerning the UV spectral distributions in the case of stars of the types B 0.5 V to A 1 V. It is found that the observed UV flux envelopes fall significantly below the envelopes predicted from lightly line-blanketed models.

Underhill, A. B.↗
NASA NTRS2015

X-Raying the Coronae of HD 155555

We present an analysis of the high-resolution Chandra observation of the multiple system, HD 155555 (an RS CVn type binary system, HD 155555 AB, and its spatially resolved low-mass companion HD 155555 C). This is an intriguing system which shows properties of both an active pre-main sequence star and a synchronised (main sequence) binary. We obtain the emission measure distribution, temperature structures, plasma densities, and abundances of this system and compare them with the coronal properties of other young/active stars. HD 155555 AB and HD 155555 C produce copious X-ray emission with log L(sub x) of 30.54 and 29.30, respectively, in the 0.3-6.0 kiloelectronvolt energy band. The light curves of individual stars show variability on timescales of few minutes to hours. We analyse the dispersed spectra and reconstruct the emission measure distribution using spectral line analysis. The resulting elemental abundances exhibit inverse first ionisation potential effect in both cases. An analysis of He-like triplets yields a range of coronal electron densities 1010 − 1013 per cubic centimeter. Since HD 155555 AB is classified both as an RS CVn and a PMS star, we compare our results with those of other slightly older active main-sequence stars and T Tauri stars, which indicates that the coronal properties of HD 155555 AB closely resemble that of an older RS CVn binary rather than a younger PMS star. Our results also suggests that the properties of HD 155555 C is very similar to those of other active M dwarfs.

stars: coronae↗
NASA NTRS1985

Protoplanetary material around nearby stars

A statistical analysis of IRAS observations indicates that about 20 percent of the nearby main sequence stars, and probably a similar fraction of main sequence stars not detectable by IRAS, shows evidence of significant infrared excess beyond 12 microns. Spectral type A dwarfs are predominant, but not exclusive in showing large 60-micron excesses. The solar system, when viewed from sufficient distance, appears to have, in contrast to this, less than 1 percent of cool infrared excess. These observational facts are consistent with the working hypothesis that cold infrared excess in stable main sequence stars is due to protoplanetary material, observable during the accretion phase in the evolution of a solar system.

Aumann, H. H.↗
NASA NTRS1993

The coronae of main-sequence A stars

We have undertaken a deep X-ray imaging survey of single, normal, mid-to-late-A- and early-F-type main-sequence stars using the ROSA T PSPC. Eight of the nine such stars in our fields went undetected at very weak emission levels, with upper limits on the normalized X-ray flux in each case of better than log R(x) below -6.3. The only A-F star we detected was 83 Tau, a member of the Hyades that we now belatedly recognize as a possible binary star, whose emission may come from an unidentified companion. Our observations reveal a systematic decline in coronal X-ray brightness with increasing effective temperature, which we attribute to the presence of strong coronal winds. These new results add further weight to earlier suggestions that single A and early-F stars in the field and in nearby older open clusters generally have intrinsically faint coronae.

Simon, Theodore↗
NASA NTRS1984

The appearance of magnetic flux on the surfaces of the early main-sequence F stars

Available chromospheric, transition region, and coronal observations of the early main-sequence F stars are examined in order to find that while these objects exhibit enhanced levels of magnetic field-related radiative emissions, significant inhomogeneities in surface activity are not present. This phenomenon is discussed within the context of the calculations published by Schmitt and Rosner (1983) for the production of flux ropes of various spatial scales at a given rotation rate at the bottom of a stellar convection zone. It is found that the spatial scales and area contrast of emergent magnetic flux in these stars that, as a class, are characterized by rapid rotation and thin convection zones are substantially reduced relative to that of the sun.

Giampapa, M. S.↗
NASA NTRS1992

Radio and X-ray emission from main-sequence K stars

A small VLA radio continuum survey of 12 selected active, main-sequence K stars is presented. The targets are uniformly distributed between spectral types K0 and K8; five are single stars, five are spectroscopic binaries, and two are visual binaries. All targets are within 25 pc and were selected based on strong X-rays, rapid rotation, or strong Ca II emission lines. Seven targets were detected at the observing frequency of 8.5 GHz (3.6 cm), while sensitive upper limits are reported for the nondetections. The radio luminosities are comparable to active M dwarfs. Radio and X-ray luminosities LR and LX fulfil a relation that was previously found for M dwarfs, with LR varies as LX over 3 orders of magnitude. We further find that the more rapidly rotating K dwarfs are stronger radio emitters, irrespective of spectral type.

Gudel, M.↗
NASA NTRS2020

Reinflation of Warm and Hot Jupiters

Understanding the anomalous radii of many transiting hot gas-giant planets is a fundamental problem of planetary science. Recent detections of reinflated warm Jupiters orbiting post-main-sequence stars and the reinflation of hot Jupiters while their host stars evolve on the main sequence may help constrain models for the anomalous radii of hot Jupiters. In this work, we present evolution models studying the reinflation of gas giants to determine how varying the depth and intensity of deposited heating affects both main-sequence reinflation of hot Jupiters and post-main-sequence reinflation of warm Jupiters. We find that deeper heating is required to reinflate hot Jupiters than is needed to suppress their cooling, and that the timescale of reinflation decreases with increasing heating rate and depth. We find a strong degeneracy between heating rate and depth, with either strong shallow heating or weak deep heating providing an explanation for main-sequence reinflation of hot Jupiters. This degeneracy between heating rate and depth can be broken in the case of post-main-sequence reinflation of warm Jupiters, as the inflation must be rapid to occur within post-main-sequence evolution timescales. We also show that the dependence of heating rate on the incident stellar flux inferred from the sample of hot Jupiters can explain reinflation of both warm and hot Jupiters. TESS will obtain a large sample of warm Jupiters orbiting post-main-sequence stars, which will help to constrain the mechanism(s)causing the anomalous radii of gas-giant planets.

Thaddeus D. Komacek↗
NASA NTRS1985

The determination of the helium abundance in main-sequence B stars

It is pointed out that the abundances of deuterium, helium, and lithium provide fundamental constraints on cosmological models. The central question is related to the compatibility of the observed abundances with big bang models of primordial nucleosynthesis, taking into account a modification by subsequent nuclear processing in stars or a modification of standard models. The present paper has the objective to assess critically the feasibility of deriving accurate helium abundances from measurements of the photospheric lines in main-sequence B-type stars. A method is established for assigning atmospheric parameters to main-sequence stars with spectral types in the range (approximately) B1-B5. It is found that an analysis of stars in distant anticenter H II regions and clusters offer an alternative method which seems capable of determining relative abundances with more than the requisite accuracy.

Wolff, S. C.↗
NASA NTRS1988

Compact radio sources in the galactic plane

The preliminary results of a search for infrared counterparts to compact continuum radio sources found within 0.5 deg of the Galactic plane are presented. Out of 75 positions searched, nine very red sources, each with a probability of less than 0.01 of being a random field star, were found. Most of the nine sources are characterized by deep silicate absorption features at 10 microns and a red energy distribution that continues to rise beyond 25 microns. Six of the sources are best explained as late O or early B stars exciting a compact H II region. The radio emission is close to that expected for the rate of emission of ionizing photons for a normal main-sequence star. Although these H II regions are compact, they are not sufficiently dusty in the immediate environment of the star to significantly reduce the ionizing flux. Three of the sources show considerably more emission than expected for a main-sequence star, indicating they are either hotter than a main-sequence star with the same luminosity or the radio emision is nonthermal and coming from a more exotic object.

Jones, Terry Jay↗
NASA NTRS1994

X-ray emission from near-main-sequence B stars

Results of a Roengten Satellite (ROSAT) X-ray survey of 12 nearby near-main-sequence B stars are presented. Objects with very low interstellar hydrogen column density were chosen to study the soft X-ray emission properties of the stars. All of the stars were detected at the 3 sigma level. Spectral fits to seven of the stars are presented, and temperatures and source emission measures are derived. The spectra and characterized by emission from gas at a temperature of about 2 x 10(exp 6) K, which is lower than that typically observed for O stars. The ratio L(sub x)/L(sub Bol) decreases sharply with spectral types later than B1, reaching values of about 10(exp -9) at B3 V; much less than the value of 10(exp -7) that holds for O stars and early-B stars. The survey includes four beta Cephei stars and two Be stars. Our observations are consistent with the finding that beta Cephei variables have softer spectra than other stars of the same spectral type. The X-ray emission measures for the stars are compared with upper and lower limits derived using mass loss rates and terminal velocities from line-driven wind theory. It is concluded that if shocks embedded in the winds are the source of the X-ray emission then a significant fraction of the winds in B stars must be hot. One to a few shocks can account for the observed emission measures. As a function of spectral type, there appears to be a transition in X-ray emission properties at about B1 to B1.5, with later type stars having a smaller L(sub x)/L(sub Bol) ratio. Coronal and shock models are discussed in regards to the BO V star tau Sco which has an anomalously high source temperature.

Cassinelli, J. P.↗
NASA NTRS1981

Results from an extensive Einstein stellar survey

The preliminary results of the Einstein Observatory stellar X-ray survey are presented. To date, 143 soft X-ray sources have been identified with stellar counterparts, leaving no doubt that stars in general constitute a pervasive class of low-luminosity galactic X-ray sources. Stars along the entire main sequence, of all luminosity classes, pre-main sequence stars as well as very evolved stars have been detected. Early type OB stars have X-ray luminosities in the range 10 to the 31st to 10 to the 34th ergs/s; late type stars show a somewhat lower range of X-ray emission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late type main-sequence stars show little dependence of X-ray emission levels upon stellar effective temperature; similarly, the observations suggest weak, if any, dependence of X-ray luminosity upon effective gravity. Instead, the data show a broad range of emission levels (about three orders of magnitude) throughout the main sequence later than F0.

Vaiana, G. S.↗
NASA NTRS1985

Stellar activity cycles

The variety of stellar chromospheric and coronal activity provides a framework for understanding solar and stellar magnetic activity. It is pointed out that the recognition of an 11-yr periodicity within the record of sunspot numbers is a relatively recent discovery of just over a century ago. The solar activity cycle is examined, taking into account the sunspot cycle, and solar dynamo models. Time-averaged stellar chromospheric activity levels are considered along with long-term activity fluctuations of cool stars. Attention is given to the direct measurement of stellar magnetic fields, solar luminosity variations, the RS Canum Venaticorum and BY Draconis variables, continuum variations in lower main sequence stars, chromospheric variations in lower main sequence stars, and chromospheric variations in evolved stars.

Baliunas, S. L.↗
NASA NTRS1995

Hydrodynamics of binary coalescence. 2: Polytropes with gamma = 5/3

We present a new numerical study of the equilibrium and stability properties of close binary systems. We use the smoothed-particle hydrodynamics (SPH) technique both to construct accurate equilibrium configurations in three dimensions and to follow their hydrodynamic evolution. We adopt a simple polytropic equation of state p = K(sub rho)(exp gamma) with gamma = 5/3 and K = constant within each star, applicable to low-mass degenerate dwarfs as well as low-mass main-sequence stars. For degenerate configurations, we set the two polytropic constants equal, K = K prime, independent of the mass ratio. For main-sequence stars, we adjust K and K prime so as to obtain a simple mass-radius relation of the form R/R prime = M/M prime, where R prime and M prime are the radius and mass of the secondary. Along a sequence of binary equilibrium configurations for two identical stars, we demonstrate the existence of both secular and dynamical instabilities, confirming directly the results of recent analytic work. We use the SPH method to calculate the nonlinear development of the dynamical instability and to determine the final fate of the system. We find that the two stars merge together into a single, rapidly rotating object in just a few orbital periods. Equilibrium sequences are also constructed for systems containing two nonidentical stars. These sequences terminate at a Roche limit, which we can determine very accurately using SPH. For two low-mass main-sequence stars with mass ratio q approximately less than 0.4 we find that the (synchronized) Roche limit configuration is secularly unstable. We discuss the implications of our results for the evolution of double white-dwarf systems and W Ursae Majoris binaries.

Rasio, Frederic A.↗