LRS Spectra Research Articles

Stellar Chromospheric Activity Database of Solar-like Stars Based on the LAMOST Low-Resolution Spectroscopic Survey

A stellar chromospheric activity database of solar-like stars is constructed based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Low-Resolution Spectroscopic Survey. The database contains spectral bandpass fluxes and indexes of Ca ii H and K lines derived from 1,330,654 high-quality LRS spectra of solar-like stars. We measure the mean fluxes at line cores of the Ca ii H and K lines using a 1 Å rectangular bandpass and a 1.09 Å FWHM triangular bandpass, as well as the mean fluxes of two 20 Å wide pseudocontinuum bands on the two sides of the lines. Three activity indexes, S rec based on the 1 Å rectangular bandpass and S tri and S L based on the 1.09 Å FWHM triangular bandpass, are evaluated from the measured fluxes to quantitatively indicate the chromospheric activity level. The uncertainties of all the obtained parameters are estimated. We also produce spectrum diagrams of Ca ii H and K lines for all the spectra in the database. The entity of the database is composed of a catalog of spectral sample and activity parameters and a library of spectrum diagrams. Statistics reveal that the solar-like stars with a high level of chromospheric activity (S rec > 0.6) tend to appear in the parameter range of T eff (effective temperature) < 5500 K, 4.3 < log g (surface gravity) < 4.6, and −0.2 < [Fe/H] (metallicity) < 0.3. This database, with more than 1 million high-quality LAMOST LRS spectra of Ca ii H and K lines and basal chromospheric activity parameters, can be further used for investigating activity characteristics of solar-like stars and the solar−stellar connection.

MONITORING OBSERVATIONS OF H2O AND SiO MASERS TOWARD POST-AGB STARS

We present the results of simultaneous monitoring observations of <TEX>$H_2O$</TEX> <TEX>$6_{1,6}-5_{2,3}$</TEX> (22GHz) and SiO J=1-0, 2-1, 3-2 maser lines (43, 86, 129GHz) toward five post-AGB (candidate) stars, using the 21-m single-dish telescopes of the Korean VLBI Network. Depending on the target objects, 7 - 11 epochs of data were obtained. We detected both <TEX>$H_2O$</TEX> and SiO maser lines from four sources: OH16.1-0.3, OH38.10-0.13, OH65.5+1.3, and IRAS 19312+1950. We could not detect <TEX>$H_2O$</TEX> maser emission toward OH13.1+5.1 between the late OH/IR and post-AGB stage. The detected <TEX>$H_2O$</TEX> masers show typical double-peaked line profiles. The SiO masers from four sources, except IRAS 19312+1950, show the peaks around the stellar velocity as a single peak, whereas the SiO masers from IRAS 19312+1950 occur above the red peak of the <TEX>$H_2O$</TEX> maser. We analyzed the properties of detected maser lines, and investigated their evolutionary state through comparison with the full widths at zero power. The distribution of observed target sources was also investigated in the IRAS two-color diagram in relation with the evolutionary stage of post-AGB stars. From our analyses, the evolutionary sequence of observed sources is suggested as OH65.5+1.3<TEX>${\rightarrow}$</TEX>OH13.1+5.1<TEX>${\rightarrow}$</TEX>OH16.1-0.3<TEX>${\rightarrow}$</TEX>OH38.10-0.13, except for IRAS 19312+1950. In addition, OH13.1+5.1 from which the <TEX>$H_2O$</TEX> maser has not been detected is suggested to be on the gateway toward the post-AGB stage. With respect to the enigmatic object, IRAS 19312+1950, we could not clearly figure out its nature. To properly explain the unusual phenomena of SiO and <TEX>$H_2O$</TEX> masers, it is essential to establish the relative locations and spatial distributions of two masers using VLBI technique. We also include the <TEX>$1.2-160{\mu}m$</TEX> spectral energy distribution using photometric data from the following surveys: 2MASS, WISE, MSX, IRAS, and AKARI (IRC and FIS). In addition, from the IRAS LRS spectra, we found that the depth of silicate absorption features shows significant variations depending on the evolutionary sequence, associated with the termination of AGB phase mass-loss.

Properties of the high-mass star forming region IRAS 22475+5939

IRAS 22475+5939 has been well studied by previous astronomers, but we can still discover new characteristics about it, using the first observations of the lines from CO J=2−1, 13CO J=2−1 and 13CO J=3−2 by the KOSMA 3m telescope. The mapping of the intensity ratio of 13CO J=3−2 and 13CO J=2−1 shows that the distribution of the temperature has two peaks, neither of which coincide with the IRAS 22475+5939 source or the center of the HII region, but rather are located at the edge of the HII region. Overlays of the Spitzer IRAC 8 μm with CO contours indicate that they are associated with each other and that the strongest polycyclic aromatic hydrocarbon (PAH) emission is at the IRAS 22475+5939 source position. The IRAS LRS spectrum at 7 μm ∼ 23 μm and the PHT-s spectrum at 2 μm ∼ 12 μm of the IRAS 22475+5939 source also exhibit strong PAH emission characteristics in the main PAH bands. The diversity of the PAH family should be responsible for the plateaus in the PAH emission in the PHT-s and IRAS-LRS spectra. Analysis and modeling in the infrared bands suggest that IRAS 22475+5939 is more likely to be a class I young stellar object. Where this is the case, the star is likely to have a temperature of Teff ∼ 9995.8K, mass ∼ 15.34 M⊙, luminosity ∼ 1.54 × 104 L⊙ and age ∼ 1.54 × 104 yr. The model shows that circumstellar disc emission is important for wavelengths between 1 and 10 μm, otherwise, the envelope fluctuates for λ > 10μm. Bipolar outflow is confirmed in the molecular cloud, and the excited star of the HII region could be the driving source of the outflow. High resolution is required for better results.

Infrared Point-Source Variability between theSpitzerandMidcourse Space ExperimentSurveys of the Galactic Midplane

We present a list of 552 sources with suspected variability, based on a comparison of mid-infrared photometry from the GLIMPSE I and Midcourse Space Experiment (MSX) surveys, which were carried out nearly a decade apart. We were careful to address issues such as the difference in resolution and sensitivity between the two surveys, as well as the differences in the spectral responses of the instruments. We selected only sources where the IRAC 8.0 μm and MSX 8.28 μm fluxes differ by more than a factor of 2, in order to minimize contamination from sources where the difference in fluxes at 8 μm is due to a strong 10 μm silicate feature. We present a subset of 40 sources for which additional evidence suggests variability, using 2MASS and MIPSGAL data. Based on a comparison with the variability flags in the IRAS and MSX point-source catalogs we estimate that at least a quarter of the 552 sources and at least half of the 40 sources in the subset are truly variable. In addition, we tentatively confirm the variability of one source using multiepoch IRAS LRS spectra. We suggest that most of the sources in our list are likely to be asymptotic giant branch stars.

Challenging the Carbon Star Dust Condensation Sequence: Anarchist C Stars

There have been several investigations of the evolution of the mid-infrared (IR) dust features in carbon star spectra based on IRAS LRS data, but these studies are somewhat contradictory. In order to understand these differences in interpretations and to develop an understanding of the carbon star dust sequence, we have reexamined 26 IRAS LRS spectra of carbon stars that have also been observed spectroscopically by ISO SWS. The low resolution and narrow wavelength coverage of the IRAS LRS data hinder determination of the effect of molecular absorptions in these spectra. This has led to incorrect estimations of the continuum levels in these spectra, which has a huge effect on the continuum-divided and continuum-subtracted spectra used to analyze trends in the shape, strength, and position of the mid-IR features. The higher resolution and broader wavelength coverage of the ISO data allow more accurate fitting of the underlying continuum. We have reassessed the trends in shape, strength, and position of the ~11 μm silicon carbide (SiC) feature and the apparent emergence of the ~9 μm feature. We find that there are no correlations between the spectral parameters. We also investigate whether any of these parameters correlate with the strength of the molecular bands; no correlation was found. Moreover, we show that the apparent 9 μm feature is probably an artifact. We discuss the implications of this study in terms of both a carbon star condensation sequence and the application of this study to the larger IRAS data set.

FBS 0137+400: A Mass-Losing AGB Carbon Mira?

The absolute K-band magnitude determined for the object FBS 0137+400 is typical for Asymptotic Giant Branch (AGB) carbon stars. The large J - K color index as well as large amplitude variability (ΔK > 0.4) and the presence of Hα in the emission indicate also that this star is a mass-losing long-period Carbon Mira variable with thick circumstellar shell. The IRAS LRS spectrum indicates the presence of a dust shell surrounding this object. Monitoring of the K-band magnitude is necessary for the determination of the pulsation period of FBS 0137+400.

Period and chemical evolution of SC stars

The SC and CS stars are thermal-pulsing AGB stars with C/O ratio close to unity. Within this small group, the Mira variable BH Cru recently evolved from spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis shows that the spectral evolution was accompanied by a dramatic period increase, from 420 to 540 days, indicating an expanding radius. The pulsation amplitude also increased. Old photographic plates are used to establish that the period before 1940 was around 490 days. Chemical models indicate that the spectral changes were caused by a decrease in stellar temperature, related to the increasing radius. There is no evidence for a change in C/O ratio. The evolution in BH Cru is unlikely to be related to an on-going thermal pulse. Periods of the other SC and CS stars, including nine new periods, are determined. A second SC star, LX Cyg, also shows evidence for a large increase in period, and one further star shows a period inconsistent with a previous determination. Mira periods may be intrinsically unstable for C/O ~ 1; possibly because of a feedback between the molecular opacities, pulsation amplitude, and period. LRS spectra of 6 SC stars suggest a feature at wavelength > 15 micron, which resembles one recently attributed to the iron-sulfide troilite. Chemical models predict a large abundance of FeS in SC stars, in agreement with the proposed association.

Properties of OH/IR stars with IRAS LRS spectra

1858 OH satellite line (1612 MHz) maser sources associated with the IRAS counterparts were assembled from the literature. Cross-correlation between collected OH/IR - IRAS sources and the recent catalogue of the IRAS LRS spectra (Kwok et al. 1997) allows us to subdivide 1024 of them (after excluding OH sources with uncertain IRAS associations) into 10 groups according to the Volk & Cohen (1989) classification scheme. We have found that sources with silicate emission form the largest group (about 57%), followed by the group with silicate absorption (only about 16%). For more sensitive individual OH maser surveys these numbers differentiate even more and the ratio between silicate emission and absorption sources reaches a value close to 6. Surprisingly, we found an association of OH maser emission with a small number of stars with carbon-rich atmospheres. The distribution of these subgroups of OH/IR stars in the IRAS colour-colour and the period-colour diagrams as well as histograms of some physical properties are presented. The galactic distribution of important parameters (periods, IRAS colours, expansion and stellar velocities) for different classes of OH/IR sources is analyzed from the point of view of the stars' evolutionary status. The correlations among the periods, colours, expansion and star velocities and the pumping efficiencies for selected subgroups of OH/IR stars are also investigated. These analyses allow us to demonstrate that a significant fraction of silicate emission group sources do not form an evolutionary sequence with the silicate absorption objects- as is suggested e.g. by the IRAS colour-colour diagram, but rather form a lower initial mass sequence, i.e. these sources will never develop sufficient mass loss to manifest silicate absorption.

100-yr mass-loss modulations on the asymptotic giant branch

We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models. The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K). We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes. The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch.

Dust features in the 10-μm infrared spectra of oxygen-rich evolved stars

We have analyzed the m UKIRT CGS3 spectra of 142 M-type stars including 80 oxygen-rich AGB stars and 62 red supergiants, with a view to understanding the differences and similarities between the dust features of these stars. We have classified the spectra into groups according to the observed appearance of the infrared features. In each case the normalized continuum-subtracted spectrum has been compared to those of the other stars to find similarities and form groups. The dust features of the AGB stars are classified into six groups: broad AGB, where the feature extends from 8 μm to about 12.5 μm with little structure; broad+sil AGB, which consists of a broad feature with an emerging 9.7 μm silicate bump; and four silicate AGB groups in which a “classic” 9.7 μm silicate feature gets progressively narrower. Likewise, the supergiant spectra have also been classified into groups, however these do not all coincide with the AGB star groups. In the supergiant case we again have six groups: featureless, where there is little or no emission above the continuum; broad Super, where the feature extends from about 9 μm to about 13 μm; and four silicate Super groups, which again show a progression towards the narrowest “classic” 9.7 μm silicate feature. We compare the mean spectrum for each group, which yields two main results. Firstly, while the “classic” silicate feature is essentially identical for both AGB stars and red supergiants, the broad features observed for these two stellar types are quite different. We suggest that the dust in these two environments follows different evolutionary paths, with the dust around Mira stars, whose broad feature spectra can be fit by a combination of alumina (Al2O3) and magnesium silicate, progressing from this composition to dust dominated by magnesium silicate only, while the dust around supergiants, whose broad feature can be fit by a combination of Ca-Al-rich silicate and Al2O3, progresses from this initial composition to one eventually also dominated by magnesium silicate. The reason for the difference in the respective broad features is not clear as yet, but could be influenced by lower C/O ratios and chromospheric UV radiation fields in supergiant outflow environments. The second result concerns the 12.5 - 13.0 μm feature discovered in IRAS LRS spectra and widely attributed to Al2O3. This feature is seen predominantly in the spectra of semiregular variables, sometime in Miras and only once (so far) in supergiant spectra. We argue that it is unlikely that this feature is due to Al2O3 or, as has more recently been suggested, spinel (MgAl2O4), but could be associated with silicon dioxide or highly polymerized silicates (not pyroxenes or olivines).

NGC 6153: a super-metal-rich planetary nebula?

We have obtained deep optical spectra of the planetary nebula NGC 6153, both along its minor axis and by uniformly scanning a long slit across the whole nebula. The scanned spectra, when combined with the nebular total H beta flux, yield integrated fluxes for all the lines (similar to 400) in our spectra, which are rich in strong recombination lines from C, N, O and Ne ions. A weak O vi lambda 3811 emission line from the central star has been detected, suggesting that the nucleus of NGC 6153 has a hydrogen-deficient surface. The optical data, together with the ISO LWS 43-197 mu m spectrum and the archival IUE and IRAS LRS spectra, are used to study the thermal and density structure and to derive the heavy-element abundances from lines produced by different excitation mechanisms. In all cases, the C2+/H+, N2+/H+, O2+/H+ and Ne2+/H+ abundances derived from multiple optical recombination lines (ORLs) are consistently higher, by about a factor of 10, than the corresponding values deduced from optical, UV or infrared (IR) collisionally excited lines (CELs), regardless of the excitation energies or critical densities of the latter. The agreement between the temperature-sensitive optical forbidden lines and the temperature-insensitive IR fine-structure lines rules out temperature fluctuations as the cause of the large difference between the ORL and CEL abundances.We present the results of a new calculation of recombination coefficients for [O II] which lead to good agreement between the observed and predicted [O II] lambda lambda 7320, 7330 forbidden line intensities if these lines are solely excited by recombination at the Balmer jump temperature. Recombination excitation is also found to be important in exciting the [N ii] lambda 5754 line, which, if unaccounted for, would lead to an overestimated [N ii] temperature from the observed (lambda 6548+lambda 6584)/lambda 5754 ratio. Analysis of a number of C ii lines arising from levels as high as 7g in the recombination ladder reveals excellent agreement between their reddening-corrected relative intensities and those predicted by recombination theory. Spatial analysis of the long-slit spectra taken along the nebular minor axis yields a varying [O iii] temperature, whereas the hydrogen Balmer jump temperature of 6000 K is approximately constant across the nebula, and is 2000-3000 K lower than the [O iii] temperature. The observed high-n Balmer line decrement indicates that the hydrogen lines arise from material having an electron density of 2000(-1000)(+2000) cm(-3), consistent with the optical and IR forbidden-line density diagnostics, which yield average line-of-sight electron densities along the minor axis varying between 2000 and 4000 cm(-3).While the He/H ratio mapped by He I and He ii recombination lines is constant within 5 per cent across the nebula, the C2+/H+ and O2+/H+ recombination-line abundances decrease by a factor of 2-3 over a radius of 15 arcsec from the centre, pointing to the presence of abundance gradients. We consider a variety of hypotheses to account for the observed behaviour of the various thermal, density and abundance diagnostics. Empirical nebular models containing two components with differing densities and temperatures are able to account for many of the observed patterns, but only if one of the components is significantly hydrogen-deficient. One such model, which gives a good fit to the observed line intensities and patterns, has 500-K H-depleted material, presumed to be evaporating from dense neutral inclusions, embedded in 9500-K material with 'normal' abundances. An alternative model, which appears more physically plausible on a number of grounds, has high-density (2x10(6) cm(-3)), fully ionized, H-deficient knots embedded in the 'normal' component, although this model fails to account adequately for the observed low (6000 K) hydrogen Balmer jump temperature. However, the observed fact that the ORLs and CELs yield heavy-element abundance ratios that are identical within the uncertainties finds no obvious explanation in the context of H-deficient knot models.

Millimeter and some near infra-red observations of short-period Miras and other AGB stars

Millimeter observations of 48 oxygen- and 20 carbon-rich AGB Miras with periods shorter than 400 days are presented. In addition, observations of 14 O-rich and 15 C-stars with longer, or no known, periods have also been obtained. The detection statistics is as follows: in 12CO and 2-1 we observed 97 stars, and detected 66 in at least one line. We find 24 new detections in the 1-0 line, 38 new detections in the 2-1 line, and 29 stars have been detected for the first time in one or both lines. In 12CO we observed 14 stars and detected 11, with 4 new detections. In 13CO , 3-2 we observed 2 stars and had one new detection. In HCN(1-0) we observed 5 carbon stars and detected 3, one new. In SO() we observed the same 5 stars and detected none. In CS(3-2) we observed 8 carbon stars and detected 3, all new. In SiO(3-2, ) we observed 34 O-rich stars and detected 25, all new except one. Near-infrared JHK photometry is presented for seven stars. For four stars it is the first NIR data published. The luminosity and dust mass loss rate are obtained for seven very red stars with unknown pulsation period from modelling the spectral energy distribution (SED) and IRAS LRS spectra. Thereby, a new IR supergiant is confirmed (AFGL 2968). For the rest of the sample, luminosity and distance are obtained in a variety of ways: using hipparcos parallaxes, period-luminosity and period-MK-relations combined with apparent K magnitudes, and kinematic distances. The dust mass loss rate is obtained from model fitting of the SED (either from the literature, or presented in the present paper), or from the observed IRAS 60 μm flux, corrected for the photospheric contribution. The gas mass loss rate is derived from the observed CO line intensities, as presented here, combined with existing literature data, if any. This allows the derivation of the dust-to-gas ratio. Our and literature CO data has been used to calibrate the relation between mass loss rate and peak intensity of the CO(3-2) line. Diagrams showing mass loss rate, dust-to-gas ratio and expansion velocity versus pulsation period are presented. Our observations confirm the existence of an upper limit for the expansion velocity of C- and O-rich stars, and that this maximum is larger for C-stars, as predicted by the theory of radiation pressure on dust particles.

OH/IR stars with SiC circumstellar envelopes

From the LRS spectra of almost 2000 IRAS sources showing the 1612 MHz OH emission we found 9 candidates for OH/IR stars with SiC features in their envelopes. Further study indicates that 6 of these are definitely or very probably carbon star with the 11.3 μm SiC emission feature in addition of being OH/IR stars showing the 1612 MHz OH maser emission.

Mid‐Infrared Spectra of Late‐Type Stars: Long‐Term Evolution

Recent ground-based mid-infrared spectra of 29 late-type stars, most with substantial dust shells, are compared to ground-based spectra of these stars from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra of about half the stars show no detectable changes, implying that their distributions of circumstellar material and associated dust grain properties have changed little over this time interval. However, many of the stars with strong silicate features showed marked changes. In nearly all cases the silicate peak has strengthened with respect to the underlying continuum, although there is one case (VY CMa) in which the silicate feature has almost completely disappeared. This suggests that, in general, an oxygen-rich star experiences long periods of gradual silicate feature strengthening, punctuated by relatively rare periods when the feature weakens. We discuss various mechanisms for producing the changes, favoring the slow evolution of the intrinsic dust properties (i.e., the chemical composition or grain structure). Although most IRAS spectra agree well with ground-based spectra, there are a number of cases in which they fall well outside the expected range of uncertainty. In almost all such cases, the slopes of the red and blue LRS spectra do not match in their region of overlap.

Infrared Observations of G0.18−0.04

The Galactic Center H(II) region, G0.18-0.04, the 'Sickle', is located where the nonthermal 'Arc' crosses the Galactic plane. The Sickle appears to be the ionized edge of a dense molecular cloud. The source of ionization has been ascribed to both the interaction of the cloud with the magnetic field of the Arc and to the hot stars in the adjacent cluster, AFGL 2004, also known as the 'Quintuplet Cluster'. This paper addresses the relative locations of the stars, the ionized and molecular gas, and the sources of gas excitation and dust heating. Using NASA's Kuiper Airborne Observatory, we have observed the far infrared forbidden lines of [S(III)] 18.7 and 33.5 micrometers, [Si(II)] 34.8 micrometers, [Ne(III)] 36.0 micrometers, [O(III)] 51.8 and 88.4 micrometers, [N(III)] 57.3 micrometers, [O(II)] 63.2 and 146 micrometers, [C(II)] 158 micrometers, and [N(II)] 205 micrometers and the adjacent continua at 11 positions around G0.18-0.04, including G0.15-0.05, the 'Pistol', in a beamsize of 40 - 60 arcsec. The electron density, the ionic abundances, and the ionization structure of the H(II) region are estimated from the doubly ionized line fluxes. The density and radiation field found in the photodissociation region (PDR) between the H(II) region and the molecular cloud are estimated from the [C(II)] and [O(I)] line fluxes and the far-infrared continuum. We compare the ionization structure and the PDR properties to shell models of H(II) regions with varying distances from their exciting stars. The agreement of observations and models indicates that the hot stars of AFGL 2004 are the likely source of ionization of the Sickle. Additional hot stars are necessary to ionize the more outlying positions. However, because of its low ionization and high PDR radiation field, the Pistol cannot be as close to AFGL 2004 as indicated by its close proximity on the sky. Instead, the Pistol is probably ionized by the luminous blue variable candidate, Pistol Source A. We estimated the extinction to the region from the distribution of the J, H, and K' magnitudes of the stars in the field that we measured from the Anglo-Australian telescope and from the IRAS LRS spectrum of AFGL 2004. The extinction is fairly uniform, with no enhancement from the molecular cloud. The strength of Brackett gamma, the 19-micrometer lines and continuum, and the IRAS 25-micrometer continuum are all consistent with the absence of a dense, foreground molecular cloud. We conclude that the H(II) region is on the near side of the dense cloud.

IR and masing properties of bright IRAS sources

Comparisons between color-color diagrams of OH/IR stars with/out IR flux and/or galactic position constraints show that little of the color dispersion is innate: the most precise colors come from high flux samples away from the Galactic Center. It is advantageous to delineate the color range of O-rich shells by only color selecting objects in the IRAS Point Source Catalog (PSC) with S(25) &gt; 100 Jy, which yields 333 objects. All but 2 of these have published LRS spectra, which often suffice to determine whether an object is carbon or oxygen rich. About 80% of them have been searched for 1612 MHz masers, and ≈70% for SiO, water and/or mainline OH masers. As bright objects, most “oddballs” have been investigated at other wavelengths too. This data, together with information gleaned from a SIMBAD search, was then used to distinguish emission-line, T Tauri and carbon stars from the predominent residuum of O-rich shells.

UIR Bands in Carbon Star Spectra

Unidentified infrared emission bands (UIR bands) have been attributed to polycyclic aromatic hydrocarbons (PAHs), which are believed to require ultraviolet radiation in order for the UIR bands to be excited. If, in addition to amorphous carbon and hydrogenated amorphous carbon (HAC) particles, PAHs are able to form in the outflows of cool carbon-rich stars (Cherchneff et al. 1991), then the weak UV radiation field from such stars would be unlikely to be able to excite the UIR bands and so the PAH species could remain undetected in the spectra of C-stars. However, cool carbon stars with hot companions might be exposed to strong enough UV radiation fields for UIR-band emission to be excited from PAHs. Buss et al. (1991) reported the detection of the 8 µm UIR-band (C-C stretch) in the IRAS LRS spectrum of HD 38218 (TU Tau), a carbon star with a hotter A2III companion. To investigate the phenomenon further, we have therefore obtained UKIRT CGS3 10 µm spectra of three carbon stars with hot companions, TU Tau, UV Aur and CS776. It was found that TU Tau showed the 11.25 µm and 8.6 µm UIR-bands (both attributed to C-H bend modes) at good contrast, while UV Aur clearly exhibited the 11.25 µm UIR band. No narrow UIR-band emission was detected in the spectrum of CS776. We have fitted these 10 µm region spectra using a χ2-minimization program equipped to fit stellar and dust emission continua together with the broad SiC feature and the narrow UIR-bands. The features seen in the spectra of TU Tau and UV Aur can be well fitted by a narrow 11.25 µm UIR-band sitting on top of a broad, self-absorbed 11.3 µm silicon carbide feature. Our results therefore provide strong support for the supposition that PAHs can form in carbon star outflows.

IRAS 13568−6232: A Quiescent Symbiotic MIRA?

The IRAS point source 13568-6232 has been suggested as a possible Vega-type system—i.e., a main-sequence A-type star with circumstellar dust emission—on the basis of its IRAS colors and LRS spectrum, and an apparent identification with the A2/5 V star HD 121918. We present broadband optical and near-infrared photometry, as well as positional measurements for HD 121918 and an alternative candidate star near the IRAS position. We deduce that the alternative (an anonymous red star) is the true optical counterpart of IRAS 13568-6232. An optical spectrum shows this object to be late M type. Its unusual near-infrared colors and 1-25 μm spectral energy distribution are similar to those of the slow symbiotic nova RR Tel.

Destruction of small IR continuum emitters

We have used IRAS survey detectors and new 8–22 μm IRA S Low Resolution Spectrometer data from the Rosette Nebula and λ Orionis H II regions to probe the physics of dust emission in high radiation environments. IRAS LRS spectra extracted from neutral regions near the Rosette nebula reveal only a continuum with no trace of UIR emission. As the H II regions are approached, the IRAS survey data reveal 12 and 25 μm emission declining together, beginning well outside of the ionized region. The 25 pin emission initially declines faster than the 12 μm emission. Well inside of the Rosette H II Region. the 25 μm emission suddenly reverses its decline and begins to increase. LRS spectra from the ionized material in this regime reveal only continuum emission at wavelengths above 14 μm. Dust emission models compared to the data indicate that in regimes of enhanced radiation the normal carriers of the emission detected by both 12 and 25 ,μm survey detectors in the diffuse interstellar medium suffer destruction. The sudden increase of 25 μm survey detector emission in the Rosette, together with the LRS spectra showing only continuum emission, suggest that this emission may arise from a thermal emitter that is normally difficult to detect. Neither a simple molecular model nor a runaway sublimation grain destruction model reproduce the simultaneous decline of 12 and 25 μm emission. We propose a plausible model in which the 25 μm carriers consist of imperfectly bound conglomerates of basic structural units. Some of these. basic structural units would be released intact upon destruction of the conglomerates as denser radiationn fields are encountered. Either cyclic carbon molecular structures or amorphous solid state particles are consistent as basic structural units in this model. IRAS-LRS UIR emission bands are observed to be restricted to dense dusty molecular regions excited by early B stars.

Detection of detached dust envelopes around oxygen-rich AGB stars

Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period.