Photospheric Emission Research Articles

First XMM-Newton observations of an isolated neutron star: RX J0720.4-3125

We present the high resolution spectrum of the isolated neutron star RXJ0720.4-3125, obtained with the Reflection Grating Spectrometer on XMM-Newton, complemented with the broad band spectrum observed with the EPIC PN camera. The spectrum appears smooth, with no evidence for strong photospheric absorption or emission features. We briefly discuss the implications of our failure to detect structure in the spectrum.

Discovery of New Candidate Vega-type Systems from [ITAL]IRAS[/ITAL] and the 2 Micron All-Sky Survey

We obtained J (1.25 μm), H (1.65 μm), and Ks (2.17 μm) photometry from the 2 Micron All-Sky Survey and 12, 25, 60, and 100 μm photometry from the IRAS Faint Source Catalog of 2834 field stars. We identified in this sample 296 main-sequence (luminosity class IV, IV-V, or V) stars. We searched the sample of main-sequence stars for excess 12 μm emission with respect to J, H, and Ks photospheric emission. We discovered eight systems with 12 μm excess emission newly reported here. These excesses are likely to be from Vega-type circumstellar dust, but small-aperture photometry is needed to confirm their nature. Preliminary models that assume the excesses are circumstellar show the dust to be located ~1-10 AU from the stars, where it is heated to terrestrial temperatures, ~200-500 K. Colder dust might also exist in the systems but below the IRAS sensitivity limit. The exception is HD 93331 (B9.5 V), previously known for its IRAS far-infrared excess.

ROSAT Observations of X‐Ray Emission from Planetary Nebulae

We have searched the entire ROSAT archive for useful observations to study X-ray emission from Galactic planetary nebulae (PNs). The search yields a sample of 63 PNs, which we call the ROSAT PN sample. About 20%-25% of this sample show X-ray emission; these include 13 definite detections and three possible detections (at a 2 σ level). All X-ray sources in these PNs are concentrated near the central stars. Only A30, BD +30°3639, and NGC 6543 are marginally resolved by the ROSAT instruments. Three types of X-ray spectra are seen in PNs. Type 1 consists of only soft X-ray emission (<0.5 keV), peaks at 0.1-0.2 keV, and can be fitted by blackbody models at temperatures 1-2 × 105 K. Type 2 consists of harder X-ray emission, peaks above 0.5 keV, and can be fitted by thin plasma emission models at temperatures of a few times 106 K. Type 3 is a composite of a bright type 1 component and a fainter type 2 component. Unresolved soft sources with type 1 spectra or the soft component of type 3 spectra are most likely photospheric emission from the hot central stars. Absorption cross sections are large for these soft-energy photons; therefore, only large, tenuous, evolved PNs with hot central stars and small absorption column densities have been detected. The origin of hard X-ray emission from PNs is uncertain. PNs with type 2 spectra are small, dense, young nebulae with relatively cool (105 K) central stars, while PNs with type 3 X-ray spectra are large, tenuous, evolved nebulae with hot central stars. The hard X-ray luminosities are also different between these two types of PNs, indicating perhaps different origins of their hard X-ray emission. Future Chandra and XMM observations with high spatial and spectral resolution will help to understand the origin of hard X-ray emission from PNs.

Optical, infrared and millimetre-wave properties of Vega-like systems—IV. Observations of a new sample of candidate Vega-like sources

Optical, infrared and millimetre-wave properties of Vega-like systems—IV. Observations of a new sample of candidate Vega-like sources

Some Characteristics of the Circumstellar Shells of Stars with Variability of the R Coronae Borealis Type

The transformation of the line and molecular spectra of R CrB from absorption to emission at the 1985 minimum is traced from spectroscopic observations, and it is concluded for the first time, from a comparison with light curves and color indices, that the rapid variation of the color index U − B by −0m.6 during fading is due exclusively to this transformation. The transformation of the spectrum can result in an increase in the star's brightness in the UBV photometric bands by about 1m.4, 0m.75, and up to 0m.75, respectively. This removes one of the main objections to using the concept of a homogeneous spherical shell to interpret the R Coronae Borealis phenomenon. High-resolution spectroscopic observations of SV Sge, a star with variability of the R Coronae Borealis type, showed the presence of an intense circumstellar component in Na I D lines with a violet shift corresponding to 240 km/sec, which is unrelated to visible brightness minima. It is suggested that a stream of matter with such a velocity through the star's vicinity is the cause of the excitation not only of the broad emission features observed during brightness minima upon fading of the photospheric emission but also of the narrow ones. Assuming that a brightness minimum is associated with the temporary formation of another spherical screening shell, some characteristics of the permanent shell are determined. Its optical depth and geometrical thickness are 0.2-0.7 and at least 0.4 of its radius, respectively.

ISOGAL survey of Baade's Windows in the mid-infrared

The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to determine their stellar content and its possible bearing on the history of the Galaxy. The NGC6522 and Sgr I Baade's Windows of low obscuration towards the inner parts of the Bulge represent ideal places in which to calibrate and understand the ISOGAL colour-magnitude diagrams. The survey observations were made with the ISOCAM instrument of the ISO satellite. The filter bands chosen were LW2(~7 microns) and LW3(~15 microns). The results presented here show that most of the detected objects are late M-type giants on the AGB, with a cut-off for those earlier than M3-M4. The most luminous members of these two Bulge fields at 7 microns are the Mira variables. However, it is evident that they represent the end of a sequence of increasing 15 micron dust emission which commences with M giants of earlier sub-type. In observations of late-type giants the ISOCAM 15 micron band is mainly sensitive to the cool silicate or aluminate dust shells which overwhelm the photospheric emission. However, in ordinary M-giant stars, the 7 micron band is not strongly affected by dust emission and may be influenced instead by absorption. The nu2 band of water at 6.25 microns and the SiO fundamental at 7.9 microns are likely contribitors to this effect. A group of late M stars has been found which vary little or not at all but have infrared colours typical of well-developed dust shells. Their luminosities are similar to those of 200-300 day Miras but they have slightly redder [7]-[15] colours which form an extension of the ordinary M giant sequence. The Mira dust shells show a mid-infrared [7]-[15] colour-period relation. ca 700 days.

A Disk Census for the Nearest Group of Young Stars: Mid-Infrared Observations of the TW Hydrae Association

A group of young, active stars in the vicinity of TW Hydrae has recently been identified as a possible physical association with a common origin. Given its proximity (»50 pc), age (»10 Myr), and abundance of binary systems, the TW Hya association is ideally suited to studies of the diversity and evolution of circumstellar disks. Here we present mid-infrared observations of 15 candidate members of the group, 11 of which have no previous flux measurements at wavelengths longer than 2 mm. We report the discovery of a possible 10 mm excess in CD 23377795, which may be due to a circumstellar disk or a faint and as yet undetected binary companion. Of the other stars, only TW Hya, HD 98800, Hen 3-600A, and HR 4796A—all of which were detected by IRAS—show excess thermal emission. Our 10 mm flux measurements for the remaining members of the association are consistent with photospheric emission, allowing us to rule out dusty inner disks. In light of these findings, we discuss the origin and age of the TW Hya association as well as the implications for disk evolution timescales. Subject headings: accretion, accretion disks — binaries: general — circumstellar matter — open clusters and associations: individual (TW Hydrae) — stars: pre‐main-sequence

Detection of a subarcsecond dust shell around the Wolf-Rayet star WR 112

A lunar occultation event of the Wolf-Rayet star WR 112 (type WC9) has been observed simultaneously from two independent telescopes at λ = 2.2 μm, allowing us to investigate this source with an angular resolution of ≈ 0.003 arcsec. We have detected a circumstellar dust envelope the brightness distribution of which can be approximately fitted by a Gaussian with a FWHM of ≈0.06 arcsec (≈ 1015> cm). We present and discuss the reconstructed brightness profile, which shows an asymmetry in the radial dust distribution. The derived dust grain temperature at the inner dust zone of ≈ 1150 K is consistent with available model calculations. There is no signature of the central star from our observations, providing a direct confirmation that the circumstellar shell emission dominates over the photospheric emission at 2.2 μm as predicted by fits to the spectral energy distribution. Further lunar occultation observations at different position angles are essential to reconstruct the two-dimensional image of the dust shell around WR 112. The current series of lunar occultations of WR 112 will continue to the end of 1999, and will be visible for all equatorial and southern latitude observatories.

Circumstellar Disks in the Orion Nebula Cluster

We combine our previous optical spectroscopic and photometric analysis of ~1600 stars located in the Orion Nebula Cluster (ONC) with our own and published near-infrared photometric surveys of the region in order to investigate the evidence for and properties of circumstellar disks. We use the near-infrared continuum excess as our primary disk diagnostic, although we also study sources with Ca II triplet emission and those designated as proplyds. The measured near-infrared excess is influenced by (1) the presence or absence of a circumstellar disk, (2) the relative importance of disk accretion and inner disk holes, (3) the relative contrast between photospheric and disk emission, and (4) system inclination. After attempting to understand the effects of these influences, we estimate the frequency of circumstellar disks and discuss the evidence for trends in the disk frequency with stellar mass (over the mass range <0.1–50 M_⊙), stellar age (over the age range <0.1–2 Myr), and projected cluster radius (over the radial range 0–3 pc). We find that the fraction of stars retaining their inner (<0.1 AU) circumstellar disks to the present time is at least 55% and probably no more than 90%, averaged over the entire range in stellar mass and stellar age represented in the ONC and over the entire area of our survey. We find no trend in the disk fraction with stellar age, at least not over the limited age range of the cluster. We find that more massive stars are less likely to have disks, consistent with a scenario in which the evolutionary timescales are more rapid for disks surrounding more massive stars than for disks surrounding less massive stars. We also find that the disk frequency begins to decrease toward the lowest masses, although objects of all masses (including those that appear to be substellar) can have disks. We find that the disk frequency increases toward the cluster center. We then argue, using several lines of evidence, that a large fraction of the disks associated with stars in the ONC are accretion disks. The observed trends with stellar age, stellar mass, and projected cluster radius in the disk frequency may, in fact, be driven primarily by trends in the disk accretion properties. From the magnitude of the near-infrared excess above that expected from pure irradiation disks, we find an accretion disk fraction among the stars identified as having disks of 61%–88%. In addition, approximately 20% of the stars in our optical spectroscopic sample show broad (several hundred km s^(-1) FWHM) Ca II emission lines, which are features often associated with accretion disk/wind phenomena; another 50% of the sample have Ca II lines that (at our spectral resolution) are filled in, indicating an independently derived accretion disk frequency of ~70%. Finally, we discuss the near-infrared and optical emission-line properties of that portion of our sample identified from Hubble Space Telescope imaging as having a dark silhouette or an externally ionized structure. This sample, proposed in the literature to have accretion disks, appears to be no different in terms of its stellar or circumstellar properties from the rest of the ONC population. The only feature distinguishing these objects from their ONC siblings thus may be their current (but short-lived) proximity to the massive stars near the cluster center.

ITAL]ROSAT[/ITAL] X-Ray Observations of Two Planetary Nebulae: NGC 1535 and NGC 3587

X-ray emission from planetary nebulae (PNs) may originate from the central stars or from the shocked fast stellar winds. These two origins may be distinguished by the spatial extent and the spectral properties of the X-ray emission. Using ROSAT Position Sensitive Proportional Counter (PSPC) observations, Leahy et al. reported X-ray emission from the PNs NGC 1535 and NGC 3587. However, a careful comparison between optical and X-ray images of NGC 1535 shows that the X-ray emission is coincident with a star outside the optical boundary of the nebula. Previous reports of X-ray emission from NGC 1535 are therefore spurious. The PSPC image of NGC 3587 shows three peaks, two projected within and one outside the optical shell boundary. To improve the spatial resolution, we have obtained a 35 ks ROSAT High Resolution Imager observation of NGC 3587. The HRI image shows clearly that the three peaks are point sources: RX J111447.9+550106, RX J111450.9+550208, and RX J111504.9+550141. RX J111447.9+550106 is projected at the position of the central star, and its PSPC energy distribution shows a very soft X-ray spectrum; therefore, RX J111447.9+550106 most likely represents the photospheric emission from the 110,000 K central star. RX J111450.9+550208 and RX J111504.9+550141 have spectral properties different from those of RX J111447.9+550106 and are probably unrelated background sources.

A Protocometary Cloud around HR 4796A?

We report both ROSAT observations and ground-based 10.8 ?m imaging of the wide binary HR 4796, which consists of a main-sequence A-type star with a large amount of circumstellar dust, HR 4796A, and, at a separation of 77, a pre-main-sequence M-type companion, HR 4796B. From the ROSAT data, we find that the X-ray emission is centered on HR 4796B, with LX/Lbol ? 3 ? 10-4. The 10.8 ?m flux, which arises from HR 4796A, displays an excess over the photospheric emission of 0.08 ? 0.02 Jy, a result consistent with the previous characterization of the emission from the circumstellar dust in the wavelength range 12 ?m ? ? ? 100 ?m as a 110 K blackbody. The Hipparcos data can be used to argue that the three main-sequence A-type stars in the Bright Star Catalogue with LIR/Lbol > 10-3 (HR 4796A, ? Pic, and 49 Cet) all have low luminosities for their colors. We argue that approximately 20% of all A-type stars pass through an early phase where they possess an amount of circumstellar dust comparable to that found around HR 4796A or ? Pic. In order to explain the result that the grain emission can be approximated by a 110 K blackbody, we propose that the circumstellar grains are largely composed of ice particles with a typical radius near 100 ?m, and that they sublimate rapidly when they are closer than ~35 AU to HR 4796A. This swarm of ice particles might be a protocometary cloud.

Infrared, submillimeter, and millimeter observations of the Soft Gamma-Ray Repeaters

The Soft Gamma-Ray Repeaters appear to be a completely new manifestation of neutron stars. SGR 1806-20 and SGR 1900+14 may have highly unusual stellar counterparts, whose spectra peak in the far infrared. They have not been detected at submillimeter or millimeter wavelengths. Their infrared spectra appear to contain several components: the photospheric emission from star(s) dominates at shorter wavelengths, a bright point source dominates at 25 μm, while an extended source dominates at 60 μm. Their spectra are inconsistent with monoenergetic synchrotron and black body radiation models, but are consistent with simple dust models. We briefly review here our latest millimeter, submillimeter, and infrared observations. We include new upper limits at 235 GHz (1.3 mm) for SGR 1806-20 and SGR 0525-66. The extended IRAS emission detected for SGR 0525-66 is consistent with that expected from heated dust in the supernova remnant N49. Here we show a preliminary analysis of our recent 9.862 μm imaging of the region around SGR 0525-66. We do not detect any point sources, at flux limits lower than would be expected if the other two SGR were placed at the distance of the LMC.

Disk Accretion Rates for T Tauri Stars

We present new measurements of disk accretion rates for T Tauri stars in the Taurus molecular cloud complex. Our results are based on intermediate-resolution spectrophotometry from 3200 to 5200 A, which is used to derive the excess hot continuum emission produced by accretion onto the central star. Previous estimates of T Tauri accretion rates in the literature differ by as much as 1 order of magnitude; our measurements agree better with the lowest estimates, and we discuss the problems and systematic effects that led to the previous disagreement. In particular, we note that the stellar photospheric emission from nonaccreting T Tauri stars exhibits color anomalies compared to main-sequence stars; these anomalies make the estimated extinction depend upon the color index used. We argue that the V-R index is a reasonable compromise to match with optically derived spectral types, and that V-I and V-J are much more likely to be biased by cooler companion stars and starspots. We develop a calibration with which approximate mass accretion rates can be derived for T Tauri stars based on broadband photometry and spectral types, which should enable accretion rates to be estimated for large samples with greater ease.

Investigation of a group of rapidly rotating a stars having an emission excess at iras wavelengths

IRAS data on 13 normal and 4 shell A stars are analyzed to detect circumstellar matter around them. Most of the stars rotate rapidly; five of them have been studied before. By comparing the observed fluxes, IUE and ground-based, with Kurucz’s models in the 0.17-0.9 µm range, the stars’ parameters were determined (\(T_{\hbox{\ \hbox{$\mid$}\kern -1em\lower .5em \hbox{$\leftarrow$}}} g_{\hbox{\ \hbox{$\mid$}\kern -1em\lower .5em \hbox{$\leftarrow$}}} \), and angular diameter), which were used to find the IR flux excesses over the photospheric emission. Infrared excesses were found in 15 of the stars. The excesses themselves are interpreted as thermal emission from dust particles with an effective temperature in the range from 70 to 200 K. The effective radius of the circumstellar disk is estimated to be from 7 to 400 R*,

EUV activity in late-type stars during the ROSAT WFC All-Sky Survey - II. Comparisons with other observations

The extreme ultraviolet (BUV) observations of the ROSAT Wide Field Camera AlI­ Sky Survey have revealed bright and energetic coronae for a sufficiently large number of main-sequence stars that detailed comparisons with other observables can be made. We have detected a large number of energetic BUV flares on stars of different spectral classes, from dF5 to dM6, including flare stars. The BUV emission is well-correlated to several stellar properties such as stellar colour, spectral type, bolometric luminosity, far-infrared photospheric emission, etc. These relations are stronger for the highly active stars like flare stars, and they become somewhat looser for the less active dwarfs. We find that there is a better correlation between the BUV luminosity and stellar rotation period for low-activity stars than for high-activity ones, and this also applies to the RS CVn stars. This is a strong indication of magnetic/active region saturation in these more active stars. We find that there is also a tendency of the L Evv to increase with magnetic field intensity, although there are very few stars with reliable magnetic field strength measurements. These last two correlations support the view that the BUV emission from these stars is regulated by magnetic processes. Our results indicate that the stellar coronae are continuously being heated by small-scale flare events.

Near‐Infrared Emission of Protostars

Models for the thermal emission from dusty infalling envelopes around protostars indicate that the envelope emission can greatly exceed the stellar plus disk photospheric emission at wavelengths ~2 μm. We argue that this thermal envelope emission accounts for the weakness of 2.3 μm CO first-overtone absorption lines in protostellar sources. Disk emission alone is unlikely to explain the observed effect, either because disks exhibit their own CO absorption, or because inner disk holes eliminate the region of the disk that can emit in the near-infrared. We find that this near-infrared veiling is very dependent on the envelope density, increasing as mass infall rate increases and centrifugal radius decreases. The veiling also depends on the characteristics of the underlying object, and it is largest when most of the luminosity is due to accretion and the disk hole size is several stellar radii. The observed veiling indicates that dust must be falling in to distances of ~0.1 AU of the central star.

Dynamic Processes in Be Star Atmospheres. V. Helium Line Emissions from the Outer Atmosphere of λ Eridani

The He I lines of the mild B2e star λ Eri often exhibit rapid, small-amplitude emissions that can occur at random places in their photospheric lines, even when the star is in a nonemission state. New simultaneous observations of the triplet λ5876 and singlet λ6678 lines show that the emission ratio for these lines is near unity, contrary to the predictions of either non-LTE model or nebular theory. Several He I emission events point to the formation of short-lived structures near the star's surface. On 1995 September 12 the line λ6678 exhibited a strong (0.13Icont) emission lasting some 20 minutes. The rapid decay of this feature implies a density of ≥1011.5 cm-3 for an emitting plasma structure near the star. This value is consistent with density estimates for slabs which may be responsible for ephemeral dimples in this star's He I lines on other occasions. We argue that photospheric helium emissions during Hα-quiescent phases are caused by foreground material and ask what mechanism might produce these features against the stellar background. To answer this question we have simulated He I line emission from model slabs having various properties and suspended over the star. We find that illumination by a source of extreme-ultraviolet (EUV) or X-ray flux depletes the He I column density so that it is difficult to form observable He I lines. A more interesting set of conditions occur for slabs with high densities (~1012 cm-3) and moderately large optical thicknesses in optical He I lines. Under these modified assumptions modest amounts of emission can be reproduced in singlet and triplet lines, and in the observed ratio. The key to producing this emission is for the slab to feel its own Lyman continuum radiation. This condition causes λ584 and other resonance lines to partially depopulate the ground state and to overpopulate the first few excited levels, ensuring that the departure coefficients of relevant atomic levels approach common values. The second necessary ingredient is a high density, which tends to equalize the departure coefficients of excited levels through recombinations and through redistribution of electrons among the l-sublevels. The combination is a kind of Lyman-pumped recombination because it relies on the Lyman continuum being marginally optically thick. Our results are consistent with studies of He I emission from planetary nebulae, symbiotic variables, and active galactic nuclei (AGNs), and may have a bearing on other detached atmospheres problems as well. This study appears to be the first application of such a mechanism to a quasi-photospheric setting.

Infrared Fabry‐Perot Imaging of M82 [Feii] Emission. II. Tracing Extragalactic Supernova Remnants

We report high spatial and spectral resolution [Fe II] 1.644 μm Fabry-Perot imaging observations of M82. We present extinction-corrected [Fe II] images and discuss the nature of compact [Fe II] emission regions revealed by these new data. We conclude that these [Fe II] sources trace a population of supernova remnants in M82 that are substantially older than those revealed previously on 6 cm radiographs. In addition, we find that M82 contains a distributed [Fe II] emission component that is extended along the southern minor axis and that accounts for 90% of the galaxy's [Fe II] luminosity. We interpret this extended emission as tracing disk material entrained in a super wind that has broken out of the galactic disk to the south. We find that the [Fe II]/Brγ line ratio throughout M82 correlates with the age of the starburst as reflected by the color of the photospheric emission from the galaxy's stars. This correlation suggests that the [Fe II] emission regions in M82 are colocated with a post-main-sequence stellar population. The engineering details of our Fabry-Perot imaging methodology are also discussed.

Preliminary Results of Optical Solar Flare Spectra Studies Using a 40-Channel Densitometer

Multi-channel CCD receivers are available for solar spectral research in the Astronomical Observatory of Shevchenko University of Kiev. The observational material considered here are 24 echelle spectrograms of the flare of July 15, 1981 and certain other disk and limb flares. Some temporal aspects of flare development are discussed, as well as properties of metal lines and the structure of the flare region. Disk and limb flare photometry indicate the full spatial coincidence of the emission volumes for all emission lines. No signs of altitude stratification for the lines of H and K Call, hydrogen, and other metals are detected. Metal flare emission is revealed in the cores of Fraunhofer lines, providing that a compact emission source is present in the active region and that broad wings of H and K CaII and hydrogen have formed. No broad wings are observed for metal lines. Two types of lower-intensity strips in the photospheric emission spectrum are detected; the first is the result of the lower temperature in sunspots, while the second is due to the weakening of photospheric radiation after it passes through the flare volume. No new spectral lines are observed in the spectrum of this second type of strip, which may be associated with a condensed layer of relatively cold plasma at the chromospheric level. Four components are always spatially coincident in the spectrum: (1) a compact source of higher intensity with broadened hydrogen and H and K Call line wings; (2) higher intensity in metal lines, but without broadened wings; (3) emission or absorption in helium lines; (4) a lower-intensity strip of photospheric emission due to its weakening after it passes through the flare volume.

ISO-SWS Observations of the Time Variability of Oxygen-righ Mira Variables

We present the first observation of variation of the infrared spectrum of circumstellar dust emission around an oxygen-rich Mira variable star, Z Cyg, over an entire light variation cycle, based on the periodic SWS01 observations. The 10 µm and 20 µm ‘silicate features’ become stronger relative to the photospheric emission at maximum that at minimum. In addition, the relative intensity of the 10 µm to the 20 µm features increases at maximum, indicating an increase of the temperature of circumstellar dust grain. A simple model analysis suggests enhanced dust formation near the photosphere around maximum, leading to a scenario that dust nucleation may have occurred near minimum and the dust grains may have subsequently grown till maximum. The strong observed variation suggests that the variability must be taken into account in the interpretation of the infrared spectra of oxygen-rich Mira variables.