Photospheric Continuum Research Articles

High‐Resolution Observations of Multiwavelength Emissions during Two X‐Class White‐Light Flares

We observed two X-class white-light flares (WLFs) on 2003 October 29 (~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope (DST) and its High-Order Adaptive Optics (HOAO) system in several wavelengths. The spatial resolution was close to the diffraction limit of DST's 76 cm aperture, and the cadence was as high as 2 s. This is the first time that WLFs have been observed in the near-infrared (NIR) wavelength region. We present a detailed study in this paper comparing photospheric continuum observations during the two events with corresponding line-of-sight magnetograms from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR) data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We also discuss several models that provide possible mechanisms to explain these continuum enhancements, especially in the NIR.

Resolving Atmospheric Structure in V471 Tauri

The K2V-DA binary V471 Tau has an extended atmosphere. By using the DA white dwarf as a probe, we can probe the atmosphere along the line of sight to the white dwarf with a spatial resolution of order 10,000 km. We observe absorption lines from C II through N V in absorption against the DA photospheric continuum. The velocities are those of gas in co-rotation with the K star.

New low-mass members of the Lupus 3 dark cloud: Further indications of pre-main-sequence evolution strongly affected by accretion

A spectroscopic survey of a small area at the center of the Lupus 3 star-forming core has revealed four new mid-to- late M-type members, including a M7.5 brown dwarf. One of the new members, classified as M5, displays prominent forbidden lines and strong H emission (EW(H)= 410 A), in addition to other permitted lines, and its luminosity is far below that of other members of the region with similar or later spectral types. We estimate a mass accretion rate rate of1:4 10 9 M yr 1 for this object, although with uncertainties that do not exclude values as low as 10 10 M yr 1 . Based on the H=(SII) ratio, the detection of HeI, and the CaII infrared triplet, we argue that most of the H emission is produced near the surface of the object, probably in accretion columns or at the base of jets, rather than in a low-density extended region. The strong emission-line spectrum superimposed on an unusually faint photospheric continuum thus seems to be a real, intrinsic feature rather than a result of the viewing geometry caused by an edge-on disk blocking the light from the central object. Other Lupus 3 late-type members also display noticeable underluminosity, all of them having EW(H) > 100 A as a result of the faint underlying continuum. We tentatively interpret these findings as evidence for the pre-main sequence evolution of objects with very low (possibly substellar) initial masses being significantly modified by accretion.

On the intensity contrast of solar photospheric faculae and network elements

Sunspots, faculae and the magnetic network contribute to solar irradiance variations. The contribution due to faculae and the network is of basic importance, but suers from considerable uncertainty. We determine the contrasts of active region faculae and the network, both as a function of heliocentric angle and magnetogram signal. To achieve this, we analyze near-simultaneous full disk images of photospheric continuum intensity and line- of-sight magnetic eld provided by the Michelson Doppler Interferometer (MDI) on board the SOHO spacecraft. Starting from the surface distribution of the solar magnetic eld we rst construct a mask, which is then used to determine the brightness of magnetic features, and the relatively eld-free part of the photosphere separately. By sorting the magnetogram signal into dierent bins we are able to distinguish between the contrasts of dierent concentrations of magnetic eld. We nd that the center-to-limb variation (CLV) of the contrast changes strongly with magnetogram signal. Thus, the contrasts of active region faculae (large magnetogram signal) and the network (small signal) exhibit a very dierent CLV, showing that the populations of magnetic flux tubes that underly the two kinds of features are dierent. The results are compatible with, on average, larger flux tubes in faculae than in the network. This implies that these elements need to be treated separately when reconstructing variations of the total solar irradiance with high precision. We have obtained an analytical expression for the contrast of photospheric magnetic features as a function of both position on the disk and spatially averaged magnetic eld strength, by performing a 2-dimensional t to the observations. We also provide a linear relationship between magnetogram signal and the =c os(), where is the heliocentric angle, at which the contrast is maximal. Finally, we show that the maximum contrast per unit magnetic flux decreases rapidly with increasing magnetogram signal, supporting earlier evidence that the intrinsic contrast of magnetic flux tubes in the network is higher.

An [ITAL]IUE[/ITAL] and [ITAL]HST[/ITAL] Archival Study of the Hot White Dwarf in the Symbiotic Variable RW Hydrae

We have carried out an ultraviolet spectroscopic study of the hot white dwarf in the eclipsing S-type symbiotic variable RW Hydrae (Porb = 370.2 days). Our study used orbital phase–resolved IUE archival low-resolution and high-resolution spectra and Hubble Space Telescope archival spectra, including all available images obtained at inferior conjunction (when the accreting hot white dwarf is in front of the red giant). This system is one of less than a handful of symbiotic variables in which the photospheric continuum from the hot white dwarf is the dominant source of light in the far-UV. We have found N V features characteristic of an outflow with velocity of approximately 170 km s-1, almost certainly associated with outflow from the white dwarf, as they are present only in those phases in which the white dwarf is visible. We have also performed the first model atmosphere synthetic spectral fit to the white dwarf–dominated far-UV continuum and found that the best fit was provided by a model with solar abundances, log g = 8, and a temperature of 41,000 K, considerably cooler than estimates of the temperature based on Zanstra techniques. We cannot exclude the possibility that the hot component is hidden by an accretion curtain and that our temperature does not characterize the accreting white dwarf's true photospheric temperature. For a measured red giant wind mass-loss rate of 1 × 10-7 M⊙ yr-1 and a wind accretion efficiency of 7%, we estimate = 7 × 10-9 M⊙ yr-1. The heating at this rate of accretion cannot maintain the white dwarf surface temperature even close to what we have derived. At the surface luminosity implied by our fitting, the required accretion rate for steady nuclear burning is a factor of 20 lower than the above accretion rate. Based on these results, we conclude that the hot component of RW Hya is a young (lower limit age of 1.6 Myr), accretion-heated white dwarf, very likely with nuclear shell burning, on its final cooling track.

Long‐Slit Observations of Extended Ciiλ1335 Emission around V854 Centauri and RY Sagittarii

We have obtained long-slit far-ultraviolet (1150-1730 A) spectra of the R Coronae Borealis (RCB) stars V854 Cen and RY Sgr near maximum light and pulsational phase zero with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The far-UV spectrum of each star shows a photospheric continuum rising steeply toward longer wavelengths and a prominent emission feature at C II λ1335. RY Sgr displays a second, but fainter, emission attributed to Cl I λ1351 (which is radiatively fluoresced by C II λ1335), but Cl I is weak or absent in V854 Cen. Most surprisingly, the C II emission of V854 Cen is significantly extended along the slit by ±25, about 6 × 103 AU at the distance of the star. The C II feature of RY Sgr exhibits no such gross extension. Nevertheless, subtle broadenings of the C II emissions beyond the point response profile suggest inner clouds of radius ~01 (250 AU) around both stars. V854 Cen is only the third RCB star after R CrB and UW Cen known to have a resolved shell.

First Observations of an R Coronae Borealis Star with the Space Telescope Imaging Spectrograph: RY Sagittarii near Maximum Light

We describe the far-UV (1140-1740 A) spectrum of the hydrogen-deficient R Coronae Borealis (RCB) star RY Sgr, obtained near maximum light (pulsational phase ~0.1) by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope. The far-UV spectrum shows a photospheric continuum rising steeply toward longer wavelengths and two prominent emission features at the shorter wavelengths: C II λ1335 and Cl I λ1351 (the latter is radiatively fluoresced by the 10 times stronger C II multiplet). We also find evidence for CO A-X 4th-positive system absorption band heads and possible weak CO fluorescent emissions pumped by C II λ1335, but the inferred column densities are low (~few times 1016 cm-2), consistent with formation in a warm (~5000 K) atmospheric layer. The detection of CO molecules, if confirmed, would be significant, because they are thought to play a key role in the dust ejection episodes of RCB stars through the initiation of molecular cooling catastrophes.

Large-amplitude periodic variations in the angular diameter of R Leonis

We report the first direct detection of long-term periodic diameter variations in a Mira variable. Angular diameter measurements of the 313-d period variable R Leonis at 833 nm and 940 nm obtained between 1996 February and 1997 June using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT) show a cyclic modulation of the apparent stellar diameter by approximately 35 per cent. The agreement between these new data and archival measurements from 1992 January suggests coherence in the modulation over a 5-yr period. Our data are consistent with recent models which suggest that, in photometric bands with only weak to moderate molecular contamination, periodic variations in stellar diameter of order 50 per cent can be maintained. The measurements indicate that the apparent stellar diameter was largest at visual phase 0.5 and that any phase shifts between the visual light curve and those at 833 and 940 nm were at most 0.05. The large offset (∼ 0.25) between the phase of the observed diameter maximum and that predicted for the photospheric continuum diameter variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the deeper continuum-forming layers.

Dynamic processes in Be star atmospheres. 2: He I 2P-nD line formation in lambda Eridani (outburst)

The He I lambda 6678 line of early Be stars generally shows violet (V) and red (R) emission whenever hydrogen alpha emission is present, but its use as a diagnostic has been handicapped by a poor understanding of the processes that drive it into emission. In an attempt to address this problem we obtained three series of eschelle spectra of the first two members of the singlet and triplet 2P-nD series of lambda Eri (B2e) during 1992 November 3-5 at Kitt Peak. During these observations lambda 6678 showed substantial emission variability in both the wings and central profile, providing an opportunity to compare its behavior with that of the lambda 4922, lambda 5876, and lambda 4471 lines. We found that the responses of the lines were different in several respects. Whereas the emissions in the V wings of all four lines scaled together, the R wing of the lambda 4922 line invariably responded with increased absorption whenever the R wing of lambda 6678 line showed increased emission. These same trends occurred within the central photospheric profiles. The R-wing behavior shows that much, but not all of the emission in lambda 6678 is caused by matter projected against the stellar disk. The excitation temperatures of the neighboring 2<SUP>1</SUP> P transitions, lambda 6678 and lambda 4922 must be greater than and less than the photospheric continuum temperature, respectively. We have investigated departures from local thermodynamic equilibrium (LTE) for the He I spectrum in a variety of ad hoc, perturbed model atmospheres. We have found only one way to cause the source function of lambda 6678 to increase so strongly, namely, by increasing the atmospheric temperature in the line formation region to 30,000 - 40,000 K. This effect was discovered by Auer and Mihalas for O3-O4 atmospheric models, but it has not been applied to active B stars. Our models suggest that lambda 6678 emission in Be stars can be used as a sensitive monitor of localized hot spots on these stars' surfaces. The energies involved in heating the active portions of the atmosphere are too high to be produced by gravitational infall. This leaves magnetically induced flares among the few known processes on the surfaces of stars capable of sustaining this energy level.

Formation of the Solar 10830 Å Line

One-dimensional hydrostatic-equilibrium models are shown here for faint, average, and bright components of the quiet Sun, and for a plage region, describing in each case how the atmosphere is stratified through the photosphere, chromosphere, and transition region up to a temperature of 105 K. The observed coronal line radiation is assumed to be the inward incident radiation at the 105 K boundary. This coronal radiation penetrates into the upper chromosphere causing sufficient helium ionization to populate the lower level of the He I 10830 Å line, producing optically-thin absorption of the photospheric continuum at 10830 Å. The amount of absorption, which is proportional to the optical thickness of the upper chromosphere in the 10830 line, depends on 1) the strength of the coronal lines at wavelengths in the He I 504 Å ionizing continuum, and 2) the density and geometrical thickness of the upper chromosphere. The computed 10830 Å line is shown for the four atmospheric models and for three values of the coronal illumination. The calculated off-limb 10830 intensity distribution shows a minimum in the low chromosphere and a maximum at roughly 2000 km above the photosphere, in general agreement with observations, indicating that this is the predominant height of the transition region over most of the solar surface.

Circumstellar Properties of S Stars. I. Dust Features

view Abstract Citations (53) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Circumstellar Properties of S Stars. I. Dust Features Chen, P. S. ; Kwok, Sun Abstract The 1435 stars currently classified as of S spectral type have been searched for association with the IRAS Point Source Catalog. Among the 835 S stars with IRAS counterparts, 149 are found to have usable IRAS low-resolution spectra. The circumstellar properties of these S stars are classified using the LRS. We find that the circumstellar properties of S stars are very similar to those of M stars. For the S stars that have LRS, silicate emission objects form the largest group. Only four objects of the whole sample show the SiC emission feature commonly found in carbon stars. Some of the S stars that show a photospheric continuum with no infrared excess may not be thermal pulsing AGB stars but are mass transfer binaries. We propose a new classification scheme for AGB stars, taking into account both the photospheric and circumstellar spectral properties. This scheme recognizes the importance of mass loss on the evolution of AGB stars. The evolutionary status of S stars is discussed in terms of their circumstellar properties, and an evolutionary scenario is proposed to reconcile the visible photospheric properties and the circumstellar properties of S stars. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173276 Bibcode: 1993ApJ...416..769C Keywords: ISM: DUST; EXTINCTION; STARS: AGB AND POST-AGB; STARS: CIRCUMSTELLAR MATTER full text sources ADS | data products SIMBAD (149)

CAPABILITIES OF THE AFGL MOSAIC ARRAY SPECTROMETER---TEN-MICRON SPECTRA OF BRIGHT INFRARED STARS

ABSTRACT A 9-14-micron slit spectrometer employing a novel mosaic array of 58 x 62 detectors is described. The spectrometer has been used on the University of Wyoming 2.3-m telescope with a 0.9-arcsec/pixel (reimaged) plate scale to obtain spectra of long-period variable stars. The instrumental spectral resolution and spatial point-spread function are characterized using spectra and spatial profiles extracted from spectral images of the variable stars. In order to evaluate the effectiveness of the data-acquisition and reduction procedures, the spectra are compared with examples found in the Atlas of IRAS Low Resolution Spectra for single representatives of three diverse object classes (oxygen-rich stars with strong silicate feature emission, carbon-rich stars with silicon carbide emission, and late-type stars exhibiting photospheric continuum radiation).

Electron beam as origin of white-light solar flares

We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H− population, and absorption of photospheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H− continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams.

2.0 to 2.4 micron spectroscopy of T Tauri stars

Velocity-resolved 2.0-2.5-micron observations of the T Tau stars T, DF, DG, DK, HL, and RY Tau, SU Aur, and GW Ori are presented. For each of these stars except SU Aur, the Brackett gamma line was detected in emission with line widths inthe range of about 130-230 km/s. The Brackett gamma line profile of SU Aur is complex, having components of both emission and absorption. The first measurement of CO band-head emission in DG Tau is reported, and it is shown that published radio continuum fluxes of young stars far exceed what could be produced in an envelope ionized by only the stellar photospheric Lyman continuum. The excess of radio emission is found to be much greater in low-luminosity sources (e.g., the T Tau stars). 54 references.

Nickel, argon and cobalt in the infrared spectrum of SN1987A: the core becomes visible

Infrared spectra of supernova 1987A taken in April and November 1987 from the Kuiper Airborne Observatory (KAO) show two distinctly different stages in the evolution of the expanding gas shell. In April both the optical and infrared spectrum originated from the hydrogen envelope, the still opaque remnant of the outer regions of the progenitor star's atmosphere. This region of the star was presumably of near solar chemical composition (although the elements heavier than helium are expected to have had an abundance from two to four times less than the Sun). Our April spectrum shows weak hydrogen lines rising above a 5,000-K photospheric continuum, in good agreement with this picture. By November, however, the spectrum of SN1987A had changed dramatically. Strong emission lines from heavy elements as well as many lines from highly excited levels of hydrogen dominated the spectrum with peak flux levels in the lines at or slightly above the level of the continuum in April. The intensity of these lines and the abundances of the heavy elements inferred from them demonstrate that the inner regions of the supernova are just now becoming visible at infrared wavelengths. These regfons are expected to contain heavy elements produced by advanced nuclear burning stages in the progenitor star (Sk–69 202) and in the shock wave that ejected all material external to the iron core. They are, of course, very deficient in hydrogen.

SPECTROPHOTOMETRY OF AS 353A, THE T TAURI STAR ASSOCIATED WITH HERBIG-HARO OBJECT 32.

ABSTRACT Using low-resolution spectra (Δλ 20 Å) we have made a spectrophotometric study of the active T Tauri star AS 353A which is obviously the source of the bipolar outflow which leads to the formation of the Herbig-Haro HH 32A, B, C, and D. Our observations cover the wavelength range 3300 Å < λ < 10,700 Å. We have determined line fluxes for the stronger emission lines. In addition we have measured the flux distribution (Fλ) of the continuum in the range 3300 Å < λ < 10,000 Å. Arguments using emission line and continuum as well as data on HH 32A make it probable that E (B -V) ≈ 0.5 for AS 353A, which is somewhat smaller than the value E (B -V) ≈ 0.69 determined earlier for HH 32A, though the present measurements do not definitely exclude the possibility that AS 353A has the same reddening as HH 32A. For E (B -V) ≈ 0.49 we estimate that AS 353A has an effective (photospheric) temperature of ≈ 5000 K; for E (B -V) ≈ 0.69 the corresponding value would be 6000 K. For both cases we show that the Mv corresponds to that of a subgiant and that AS 353A definitely lies above the main sequence. The AS 353A continuum shows a strong ultraviolet excess (indicating strong chromospheric radiation) with the total continuum being approximately a factor 2 larger than the photo spheric continuum at λ ≈ 3600 Å.

Epsilon Aurigae during eclipse - IRAS observations of the cool secondary component

Measurements of Epsilon Aur made from 5 to 60 microns over a period of less than 1 month during full eclipse, including special Infrared Astronomy Satellite (IRAS) observations at 12, 25, and 60 microns are presented. The observed flux at 60 microns agrees well with a prediction based on ground-based infrared monitoring of the eclipse ingress in 1982. Fits to the flux in excess of simple models for the primary star's infrared photospheric continuum yield T = 475 + or - 50 K, Omega = 8.6 + or - 1.0 x 10 to the -16th sr for the secondary. The total observed luminosity from the secondary is no larger than 50 solar luminosities.

Winds from rotating, magnetic, hot stars. I - General model results

view Abstract Citations (68) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Winds from rotating, magnetic, hot stars. I. General model results. Friend, D. B. ; MacGregor, K. B. Abstract It is generally thought that the winds of hot stars are driven by the force arising from the absorption and scattering of photospheric continuum radiation by ultraviolet resonance lines of abundant ions in the flow. However, puzzling discrepancies were found in a comparison of observational data with the predictions of theoretical models for line-driven mass loss. It is pointed out that part of the apparent disagreement between theory and observation might result from the existence of additional, nonradiative forces which contribute to the acceleration of hot star winds, but which are omitted from the model considered by Castor et al. (1975). The present investigation takes, therefore, into account the dynamics of winds which are predominantly radiation driven, but which are also acted upon by the forces due to rapid rotation in the presence of a stellar magnetic field. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162238 Bibcode: 1984ApJ...282..591F Keywords: Hot Stars; Magnetic Stars; Stellar Models; Stellar Winds; Early Stars; Magnetic Fields; Radial Velocity; Stellar Mass Ejection; Stellar Rotation; Velocity Distribution; Astrophysics full text sources ADS | data products SIMBAD (3) Related Materials (3) Part 2: 1985NASCP2358..142F Part 3: 1987ApJ...312..659M Part 4: 1992A&A...256..141M

The ultraviolet spectra of three N-type carbon stars

view Abstract Citations (26) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The ultraviolet spectra of three N-type carbon stars. Johnson, H. R. ; Obrien, G. T. Abstract The paper reports the successful detection of the ultraviolet spectra of three N-type carbon stars: BL Ori, TX Psc, and T Ind. Exposures of approximately 2-3 hours with the LWR camera of the IUE Satellite at low dispersion were necessary to record the ultraviolet flux. The shortward extension of the photospheric continuum is well exposed down to about 2800 A, with strong molecular and atomic absorption features. The spectrum appears not to be seriously affected by the unknown opacity source which blankets N stars (especially redder ones) in the violet. Emission lines of Mg II (h and k) and C II (2325 A) are conspicuous, indicating the presence of chromospheres in the stars. The ratios of integrated fluxes in the Mg II lines and C II lines to bolometric flux are respectively much smaller and slightly smaller than values observed in oxygen-rich M giants and supergiants. Publication: The Astrophysical Journal Pub Date: February 1983 DOI: 10.1086/160737 Bibcode: 1983ApJ...265..952J Keywords: Carbon Stars; Chromosphere; Stellar Atmospheres; Stellar Spectra; Ultraviolet Spectra; Atomic Spectra; Continuous Spectra; Emission Spectra; Iue; Molecular Absorption; Ultraviolet Absorption; Variable Stars; Astrophysics full text sources ADS | data products SIMBAD (8) MAST (1) INES (1)

Gamma radiation and photospheric white-light flare continuum

Recent gamma-ray observations of solar flares have provided a better means for estimating the heating of the solar atmosphere by energetic protons. Such heating has been suggested as the explanation of the continuum emission of the white-light flare. We have analyzed the effects on the photosphere of high-energy particles capable of producing the intense gamma-ray emission observed in the 1978 July 11 flare. Using a simple energy-balance argument and taking into account hydrogen ionization, we have obtained the following conclusions: (1) Heating near τ5000 = 1 in the input HSRA model atmosphere is negligible, even for very high fluxes of energetic particles. (2) Energy deposition increases with height for the inferred proton spectra, and does not depend strongly upon the assumed angle of incidence. The computed energy inputs fall in the range 10–100 ergs (cm3 s)−1 at the top of the photosphere. (3) H− continuum dominates for column densities as small as 1022 cm−3, but at greater heights hydrogen ionizes sufficiently for the higher continua to dominate the energy balance. (4) The total energy deposited in the ‘photospheric’ region of H− dominance could be within a factor of 3 of the necessary energy deposition, by comparison with the white-light flare of 1972 August 7, but the emergent spectrum is quite red so that the intensity excess in the visible band is insufficient to explain the observations.