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At least 217 records · Page 12

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NASA NTRS1993

Density wave triggered star formation in grand design spirals

In normal spiral galaxies the arms are the main sites for star formation. This is the cause of their optical contrast compared with the rest of the disc. The spiral structure can be observed as a higher concentration of H2 regions, neutral gas (both atomic and molecular via CO), dust and stars than in the interarm disc. It seens generally accepted that, at least in grand design spirals, there are density waves in the discs. However, several questions are not clear yet and still under discussion. An important question could be termed the triggering dilemma (by analogy with the 'winding dilemma' raised in the forties): Is the enhanced star formation in the spiral arms triggered by the passage of a system of density waves or is it simply due to the presence of a higher column density of gas there? In the present work, we use triggering in the same sense as the moderate to strong triggering defined by Elmegreen (1992), that is to say that star formation in the arms occurs at a rate faster than that in the interarm zone, relative to the available placental gas. Our group has designed several tests to elucidate whether or not star formation is triggered in the arms with respect to the interarm region and we summarize one of them, that of the ratio of the star formation efficiency in the arms divided by that of the interarm zone at the same galactocentric distance which we may call the relative massive star formation efficiency, where the efficiency is defined using the ratio of the mass of stars (evaluated via the H alpha flux) to the mass of neutral gas, atomic plus molecular (which must be measured with the adequate angular resolution). If the relative efficiency is of order unity, the star formation is proportional to the mass of gas, if some kind of induced star formation is present, the relative efficiency should be considerably larger than unity.

Cepa, J.↗

NASA NTRS1999

The ISO View of Star Forming Galaxies

ISO studies of normal galaxies in the local Universe have revealed basic new properties whose significant implications for the star formation process and cosmology are only starting to be understood. This review will touch on the general results of a statistical nature, and provide a quick summary of the profusion of exciting results on individual objects. In the mid-infrared, PHT-S has established that the spectra of star forming galaxies between 6 and-13microns are dominated by the Aromatic Features in Emission (AFE), and show little variation as a function of the heating intensity. The Carriers of the AFE (CAFE) are thus a universal component of dust with standard properties, and contribute between 10 and 25% of the total dust luminosity. In addition to AFE, the spectra show a low-level continuum detectable at wavelengths longer than 3.5microns whose origin is still under investigation. The mid-infrared colors formed as the ratio of flux densities in the 6.75micron and the 15micron bands of ISO-CAM remain essentially constant and near unity for quiescent and mildly active galaxies. As dust heating increases further, the 15micron flux increases steeply compared to 6.75microns, indicating that dust heated to 100K<T<200K becomes a significant component. Such hot dust is presumably located within or just outside HII regions. At the other end of the spectrum, photometry at 120-200microns using ISO-PHOT is starting to constrain the distribution of dust temperatures at the low end of the temperature scale. From LWS data, the far-infrared fine-structure lines of [CII] and [OI], which provide most of the cooling for warm atomic gas, show variations that have remained controversial in their interpretation. In particular, as the galaxy become more active in star formation, its [CII] flux weakens relative to total dust emission while the [OI] does not. This behavior has attracted much interest because it extrapolates to the most active galaxies, making them weaker in [CII] than previously expected. Several explanations for the effect have been advanced, and will be discussed in this review. Spectroscopy with SWS has measured molecular hydrogen in galaxies, providing a powerful handle on the warm molecular gas content. SWS and CAM-CVF studies targeting ionic fine-structure lines have demonstrated their value as diagnostics of the radiation field.

Helou, George↗

NASA NTRS1992

Elemental abundances of the B and A stars Gamma Geminorum, 7 Sextantis, HR 4817, and HR 5780

Fine analyses of the B and A stars, Gamma Geminorum, 7 Sextantis, HR 4817, and HR 5780 are performed. Although the data cover rather limited spectral regions, still useful results were obtained. The data were mostly obtained at the KPNO coude feed telescope with CCD TI No. 3, camera 5, and grating A. The He/H ratio of HR 4817 confirms the similarity of many abundance values with those of the peculiar Mn star 53 Tauri. For the most part Gamma Gem, 7 Sex, and HR 5780 have derived abundances similar to those of other normal sharp-lined stars of similar effective temperature.

Adelman, Saul J.↗

NASA NTRS2023

Earth/Moon System Star Tracking Sensor for Improved Deep Space Laser Communications

The goal of this research is to improve deep space communication links by exploring a new concept in stellar navigation that uses images of the Earth/Moon system to obtain precise pointing information so that the laser signal from the satellite can be directed at the ground station with unprecedented accuracy. Currently, the concept for satellite pointing relies on a dual axis star tracker that compares images of the celestial sphere to a star catalog to obtain a 3-axis orientation solution. Most star trackers are designed with a wide field of view (required to obtain enough bright starts to achieve a navigational fix) and only provide reliable pitch and yaw estimates thus necessitating the use of a second star tracker looking at an axis perpendicular to the first one to obtain an accurate estimate of the roll of the spacecraft. In the new concept the optical design will be modified to ensure that both the Earth and the Moon will be within a narrow field of view centered on Earth relative to the field of view of most star tracking systems. Because of the proximity of the bodies, it is possible to obtain highly accurate estimates of the pitch, yaw and roll of the spacecraft relative to the Earth/Moon system. The new tracker will allow for accurate 3-axis control of the spacecraft from a single tracker thus allowing the second star tracker, which would normally be pointed in a direction perpendicular to the one pointed at earth, to obtain the spacecraft roll estimate. This wide field star tracker facilitates functions like the initial satellite pointing during the acquisition phase of establishing the laser communication link or solving the lost in space problem should the system need to be reset. This new design will be tested in simulation and compared for 3-axis pointing accuracy against a standard commercial star tracker.

Star tracker↗

NASA NTRS1981

Theory of stellar coronae - An interpretation of X-ray emission from non-degenerate stellar sources

It is shown that the acoustic wave heating theory of stellar coronae explains neither Einstein Observatory coronae data nor previous UV and X-ray observations of the sun and other stars, on the evidence of data implying that magnetic fields, stellar rotation rates and convection zone parameters figure in the determination of coronal heating. Einstein Observatory results suggest that O-type star coronae are heated by the interaction of turbulent stellar winds with slowly-decaying primordial magnetic fields or by radiative instabilities in the flow. The apparent absence of coronae in Ap stars is due to the stability of atmospheres in which even weak convection is suppressed by the strong field. Dynamo action is implicated in some normal A-type stars and in F- and later-type dwarfs. Coronal characteristics of dMe and dM stars, close binaries, and K- and M-type giants are also considered.

Linsky, J. L.↗

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 NTRS1981

Autonomous attitude determination from star data for a dual-spin planetary spacecraft

A batch mode process which identifies three reference stars within a rotor-mounted star scanner's field-of-view based on the criteria of intensity and geometry was established. The sequential mode which continuously tracks the reference stars provides star transit times and estimates of rotor's spin rate. A least-square estimator was formulated which sequentially determines the spacecraft attitude from sucessive star crossings by minimizing the error in the star and scanner slit normal orthogonality. This spacecraft attitude also provides intermittent updates for the gyro propagated inertial attitude of the despun science platform. Simulation results are presented, showing successful star identification and attitude convergence in the presence of nutation and star transit time uncertainty.

Wong, E. C.↗

NASA NTRS1988

An improved photometric analysis of SX Aurigae

A new analysis of existing photometric data on SX Aurigae is presented which changes current conclusions in the literature concerning the star. It is concluded, based on fitting of the continuous spectrum to Kurucz model atmospheres and simplex optimization procedure utilized, that the system consists of two normal main-sequence stars of spectral types B2 and slightly earlier than B5. The components are very close to the inner Roche limiting surface. The secondary component is unevolved, and the primary component is extremely close to contact with its inner Roche lobe. The evidence indicates that mass flow from the primary to the secondary has not yet begun. Allowing the bolometric albedo of the secondary to be a free parameter produces a solution with an excellent fit to the B, V data. The derived value, A2 = 0.40, disagrees with the theoretical value for radiative atmospheres and agrees with the value for convective equilibrium.

Linnel, Albert P.↗

NASA NTRS1992

Silicon abundances in population I giants

Silicon to carbon abundance ratios for population I giants were determined from emission lines originating in the transition layers between stellar chromospheres and coronae. For effective temperatures larger than 6200 K we find a group of stars with increased silicon to carbon but normal nitrogen to carbon abundance ratios. These stars are presumably descendents from Ap stars with increased surface silicon to carbon abundance ratios. For G stars this anomaly disappears as is to be expected due to the increased depth of the convection zone and therefore deeper mixing which dilutes the surface overabundances. The disappearance of the abundance anomalies proves that the anomalous abundances observed for the F giants are indeed only a surface phenomenon. It also proves that the same holds for their progenitors, the Ap and Am stars, as has been generally believed. Unexplained is the increased silicon to carbon abundance ratio observed for several stars cooler than 5100 L. RS CVn and related stars do not show this increased abundance ratio. There are also some giants which appear to be enriched in carbon, perhaps due to a helium flash with some mixing if the star is a clump star.

Boehm-Vitense, Erika↗

NASA NTRS1993

Bright near-infrared sources within 1 deg of the Galactic center. I - Survey and 1-20 micron photometry

Results of a near-IR survey of 0.55 sq deg toward the Galactic center are reported. Additional IR photometry of 50 objects found in this survey was made in order to investigate the nature of luminous stars in the central region of the Milky Way including all sources with K less than 7.6 and H-K not less than 1.4. In addition to candidates for normal M-type stars and long-period variables, four objects whose energy spectra peak at about 5 microns have been detected in a small region around (l, b) = (0.15, 0.0 deg), near the crossing of the 6-cm radio arc with the Galactic plane. These four might be young stellar objects near the Galactic center. It is suggested that the relative depth change in the silicate absorption is localized to a fairly small region around the Galactic center.

Nagata, Tetsuya↗

NASA NTRS1985

IUE observations of amorphous hot galaxies

Blue amorphous galaxies are star-forming, irregularlike systems which lack the spatially distinct OB stellar groups that are characteristic of most late-type galaxies. In order to better understand the nature of star-formation processes in these unusual galaxies, short-wavelength IUE spectra of the amorphous galaxies NGC 1705 and NGC 1800 have been obtained. It is found that NGC 1705 contains a normal mix of OB stars, which is consistent with the nearly constant recent star-formation rate inferred from new optical data. NGC 1800 is likely to have similar properties, and blue galaxies with amorphous structures thus do not show evidence for anomalies in stellar populations. The UV spectra of these galaxies and a variety of other hot extragalactic stellar systems in fact have similar characteristics, which suggests OB stellar populations are often homogeneous in their properties.

Lamb, S. A.↗

NASA NTRS1982

R associations. VI - The reddening law in dust clouds and the nature of early-type emission stars in nebulosity from a study of five associations

Positions, identification charts, UBVRIKLMN photometry and spectral types are given for stars, illuminating reflection nebulae that are visible on the POSS prints, which have been identified in five associations. With a ratio of total to selective extinction of 4.2, the reddening law applicable to the dust clouds in which the stars are embedded is steeper than normal. The five associations exhibit 18 early-type stars with circumstellar shells, of which those with spectral types earlier than B5 characteristically have weak IR excesses, in contrast to the strong excesses indicative of circumstellar dust, of later-type stars. Color-magnitude charts show a distribution lying above the ZAMS by up to about 2 mag for both the circumstellar shell stars and those classified as rapid rotators. It is suggested that (1) rapid rotation accounts for the scatter in the color-magnitude diagram, and (2) many of the nebulous early-type emission-line stars are rapid rotators rather than pre-main sequence objects.

Herbst, W.↗

NASA NTRS1989

An alternative binary model for SN1987A

A class of models for SN1987A is proposed in which the blue supergiant Sk -69 deg 202 is the progenitor and where the supernova evolution of this star is strongly influenced by the accretion of matter from a companion star which was initially the more massive star. Binary stellar evolution calculations show that a model of this type can naturally explain why the apparent progenitor was a blue supergiant rather than a red one. The system became unbound after the supernova event, and the presence of a normal or neutron-star companion will probably be revealed a few years after the explosion. A model of this type can explain all the major observational features of SN1987A and also provides plausible explanations for many of its anomalies.

Podsiadlowski, PH.↗

NASA NTRS1981

A UBV photometric study of the 5.2 hour X-ray binary 4U 2129+47

The paper presents results derived from UBV photometric observations of the 5.2 X-ray binary 4U 2129+47 which were made on 11 nights between 1978 October and 1979 August. The light curves observed during this period are relatively stable in shape and amplitude (Delta B = 1.5 mag) and are a surprisingly close match to the time-averaged light curves observed for HZ Her. There is a significant asymmetry in their shape which is independent of color and varies in amplitude by a factor of approximately 2. The UBV colors at minimum light suggest a considerably earlier spectral type (G or earlier) than is inferred by assuming that the star fills its Roche lobe (late K or early M). Optical pulsations were studied, and a 3 sigma limit of 0.5% was set on the fraction of the flux which is pulsed for periods between 60 ms and 300 s. Assuming that the optical companion is a normal main-sequence star filling its Roche lobe, its mass is approximately 0.65 solar mass. Assuming also that the compact object has a mass of 1.3 solar masses and that the orbital inclination of the system is approximately 90 deg, it is found that the orbital separation is approximately 1.9 solar radii and K-opt is approximately 300 km/s.

Mcclintock, J. E.↗

NASA NTRS1982

Ultraviolet, visual, and infrared observations of the WC7 variable HD 193793

Low-resolution IUE data are used to explore the ultraviolet extinction toward the Wolf-Rayet star HD 193793 and to search for ultraviolet variability that might relate to the infrared variability. High-dispersion IUE observations are used to investigate the nature of the stellar wind of the star and to search for anomalies in the interstellar line spectrum that might be expected to be found toward a star that has recently formed a dust shell. Finally, the ultraviolet and new visual and infrared data are combined to investigate the full energy distribution of this unusual source. The energy distribution is found to extend from 0.12 to 12.5 microns, and the ultraviolet data suggest a normal WC-7 type star. A wind terminal speed of about 3000 km/s is implied by the data, as well as an E(B-V) value of 0.85. The dereddened ultraviolet to visual energy distribution is consistent with a star having effective temperature of about 43,000 K.

Fitzpatrick, E. L.↗

NASA NTRS1993

The velocity dispersion of the caustic network due to random motion of individual stars in the lensing galaxy

We present a method of computing the velocity distribution of the caustic network due to the random motion of stars in the lensing galaxy. This method is illustrated on the example of the two-point mass lens and then applied to a large sample of stars. We conclude that the proper motion of the stars increases significantly the frequency of the high magnification events in comparison with a static lens configuration with constant stream or constant bulk velocity. The stream velocity is the velocity of the star field relative to the global bulk velocity of the galaxy. We show that the global bulk and the stream velocity of the star field have to be considered separately for any microlensing situation. The higher the surface mass density of the stars in the lensing galaxy, the higher the influence of proper motion of stars on the statistics of high magnification events. The influence of a Gaussian velocity distribution of the stars in the lensing galaxy compared with a constant stream velocity of the stars increases the number of high magnification events by a factor 1.30 +/- 0.06 for a normalized surface density of the stars cr = 0.1 and by a factor 1.7 +/- 0.1 for sigma = 0.5. This means that for some microlensing situations the proper motion of the stars in a lensing galaxy has to be considered for exact microlensing predictions.

Kundik, Tomislav↗

NASA NTRS1998

An Extraordinary Cluster of Massive Young Stars in the Milky Way's Nucleus

The mass distribution of newborn stars is key to the evolution of galaxies, as it determines whether a galaxy's interstellar medium is funneled predominantly into dim, long-lived, low-mass stars, as is the case in normal galactic disks, or into bright, short-lived, massive stars, as is perhaps the case in starburst nuclei.

milky way stars galactic disks stellar clusters↗

NASA NTRSDate not supplied

Swift Mission Gyro Patch and Re-Calibration Without A Calibration Campaign

The Swift spacecraft has three Two-Axis-Rate-Assemblies (TARAs) (designated as TARA 1, TARA 2, and TARA 3, and generically referred to as gyros) which until March of 2024 were all used in rate estimation as an operational workaround (dubbed “corrector-gyro”) of a software defect discovered in 2007. TARA 1 performance degraded throughout 2023 and 2024, making it an unreliable rate source. It was determined that shifting TARA 1 to be the corrector-gyro and using mainly TARAs 2 and 3, with the existing flight software (FSW) defect, would not meet performance requirements for science operations. Rather the FSW was patched to remove the defect found in 2007. This patch was delivered by the vendor in 2009, but was not fully tested or installed at that time. A decision was made in 2024 to install the patch despite limited simulation and testing capabilities. The problem facing the team was to come up with a set of alignment parameters that would work with the new patch, using data from the spacecraft operating normally in the old configuration. Calibration activities would have been possible in either the old or new FSW configuration, but were ultimately deemed unnecessary, saving staff time and pre-serving time on the spacecraft for science data collection. This paper describes the old and new FSW configurations related to gyro processing and alignment, and how the raw gyro and star tracker data from normal operation were used with a multidimensional unconstrained nonlinear minimization in MATLAB (Nelder-Mead1) to produce a fully calibrated set of gyro alignment parameters compatible with the patched FSW. Flight data is presented showing that not only did the process work, it improved performance significantly, rivaling the best slew performance of the mission to date.

MATLAB↗