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Near-infrared photometry and spectroscopy are obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with mag. Where available, Galaxy Evolution Explorer photometric data are used to better constrain the overall spectral energy distribution of the white dwarfs, enabling any excess near-infrared flux to stand out more readily against the expected stellar photosphere. With superior data, a near-infrared photometric excess is confirmed at three metal-rich white dwarfs and ruled out at nine others. Several new binaries are confirmed or suggested; five white dwarf–red dwarf pairs and five double degenerates. Four apparently single magnetic white dwarfs – two DA and two DQp – display modest to strong near-infrared excess (relative to non-magnetic models), which may be better described as two effective temperatures owing to a redistribution of energy in highly magnetic or peculiar atmospheres. [ABSTRACT FROM AUTHOR]

Dust radiative transfer models are presented for 60 carbon stars in the Magellanic Clouds (MCs) for which 5–35 μm Spitzer infrared spectrograph (IRS) spectra and quasi-simultaneous ground-based JHKL photometry are available. From the modelling, the luminosity and mass-loss rate are derived (under the assumption of a fixed expansion velocity and dust-to-gas ratio), and the ratio of silicon carbide (SiC) to amorphous carbon (AMC) dust is also derived. This ratio is smaller than observed in Galactic carbon stars, as has been noted before. Light curves for 36 objects can be retrieved from the massive compact halo object (MACHO) and optical gravitational lensing experiment (OGLE) data bases, and periods can be derived for all but two of these. Including data from the literature, periods are available for 53 stars. There is significant scatter in a diagram where the mass-loss rates are plotted against luminosity, and this is partly due to the fact that the luminosities are derived from single-epoch data. The mass-loss rates for the MC objects roughly scatter around the mean relation for Galactic C-stars. The situation is better defined when the mass-loss rate is plotted against pulsation period. For a given period, most of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) stars have mass-loss rates that are in agreement with that observed in Galactic carbon stars (under the assumption that these objects have an expansion velocity and dust-to-gas ratio typical of the mean observed in Galactic carbon Miras). For some SMC sources only, the IRS spectrum at longer wavelengths falls clearly below the model flux predicted by a constant mass-loss rate. An alternative model with a substantial increase of the mass-loss rate to its present-day value over a time-scale of a few tens of years is able to explain the spectral energy distribution (SED) and IRS spectra of these sources. However, the probability to have two such cases in a sample of 60 is small, and makes this not a likely explanation (and testable by re-observing these objects near the end of the lifetime of Spitzer). Alternative explanations are (ad hoc) changes to the dust emissivity at longer wavelengths, and/or deviations from spherical symmetry. [ABSTRACT FROM AUTHOR]

We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1–5 μm Imager Spectrometer (UIST) in conjunction with the half-waveplate module IRPOL2. We present additional 450- and 850-μm photometry data obtained with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope ( HST) F606W- and F814W-filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and south-east. These features are not perpendicular to each other and could signify a recent ‘twist’ in the outflow axis. We model the CSE using an axisymmetric light scattering (als) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport (dart) code to fit the spectral energy distribution. A good fit was achieved with theals anddart models using silicate grains, 0.1–0.4 μm with a power-law size distribution of a−3.5, and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined to be B1i supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity is ∼4500 L⊙ and the mass of the CSE is ∼0.2 M⊙. We also determine that the mass loss lasted for ∼5300 yr with a mass-loss rate of ∼3.4 × 10−5 M⊙ yr−1. [ABSTRACT FROM AUTHOR]

We have carried out JHK polarimetric observations of 11 dusty young stars, by using the polarimeter module IRPOL2 with the near-infrared camera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Our sample targeted systems for which UKIRT-resolvable discs had been predicted by model fits to their spectral energy distributions. Our observations have confirmed the presence of extended polarized emission around TW Hya and around HD 169142. HD 150193 and HD 142666 show the largest polarization values among our sample, but no extended structure was resolved. By combining our observations with Hubble Space Telescope (HST) coronographic data from the literature, we derive the J- and H-band intrinsic polarization radial dependences of the disc of TW Hya. We find the polarizing efficiency of the disc is higher at H than at J, and we confirm that the J- and H-band percentage polarizations are reasonably constant with radius in the region between 0.9 and 1.3 arcsec from the star. We find that the objects for which we have detected extended polarizations are those for which previous modelling has suggested the presence of flared discs, which are predicted to be brighter than flat discs and thus would be easier to detect polarimetrically. [ABSTRACT FROM AUTHOR]

In order to study the inner parts of the circumstellar material around optically faint infrared bright objects, we present the first medium-resolution spectropolarimetric data taken in the near-infrared. In this paper, we discuss Paβ line data of GL 490, a well-known embedded massive young stellar object, and of MWC 349A and MWC 342, two optically faint stars that are proposed to be in the pre-main-sequence phase of evolution. As a check on the method, the classical Be star ζ Tau, known to display line polarization changes at optical wavelengths, was observed as well. Three of our targets show a ‘line effect’ across Paβ. For ζ Tau and MWC 349A, this line effect is due to depolarization by a circumstellar electron-scattering disc. In both cases, the position angle of the polarization is consistent with that of the larger scale discs imaged at other wavelengths, validating infrared spectropolarimetry as a means to detect flattening on small scales. The tentative detection of a rotation in the polarization position angle at Paβ in the embedded massive young stellar object GL 490 suggests the presence of a small-scale rotating accretion disc with an inner hole – similar to those recently discovered at optical wavelengths in Herbig Ae and T Tauri stars. [ABSTRACT FROM AUTHOR]

Echelle spectroscopy of H2 2.122 μm,[Fe ii] 1.644 μm and Brγ line emission from a very young planetary nebula (PN), IRAS 21282+5050, and from four protoplanetary nebulae (PPN), IRAS 19343+2926 (M 1−92), IRAS 17150−3224 (AFGL 6815), IRAS 17423−1755 (Hen 3−1475) and IRAS 17441−2411, is presented. H2 line emission is detected in discrete shock fronts in the lobes of each nebula, regardless of source spectral type[although non-detections in IRAS 09371+1212 (Frosty Leo) support claims that late spectral types do not produce bright H2 line emission]. In IRAS 17150−3224, we also uncover possible signs of rotation, as would be expected if the H2 features were excited in a magnetocentrifugal disc wind.[Fe ii] 1.644 μm emission was detected in only one source, M 1−92 (notably, the source with the brightest H2 features). Again, the emission is predominantly excited in high-velocity shocks in the bipolar lobes of the PPN. The H2 and[Fe ii] observations of M 1−92, and the complex H2 profiles in IRAS 21282+5050, are explained using the shock models of Smith and collaborators. We show that bow shock models are generally able to account for the observed line profiles, peak velocities, the double-peaked profiles in IRAS 21282+5050, and the spatial distribution of H2 and[Fe ii] in M 1−92. J-type bow models are adequate in each case, i.e. a strongly magnetized wind is not required.Finally, Brγ is detected in each of the five targets; in absorption in the G-type PPN, though in emission in the O- and B-type sources. Brγ emission is detected predominantly towards the near-infrared (near-IR) continuum peak in each PPN, with only very weak emission detected in the extended lobes of the O- and B-type sources. In Brγ, low peak velocities, though very broad profile widths, are measured in each target, regardless of the nebula inclination angle. The emission must therefore derive from ionized regions in a fast wind very close to the central star (rather than from shocks in the bipolar lobes), or, in the late-type sources, from absorption in an equatorial torus. [ABSTRACT FROM AUTHOR]

Echelle spectroscopy of H2 2.122 μm,[Fe ii] 1.644 μm and Brγ line emission from a very young planetary nebula (PN), IRAS 21282+5050, and from four protoplanetary nebulae (PPN), IRAS 19343+2926 (M 1−92), IRAS 17150−3224 (AFGL 6815), IRAS 17423−1755 (Hen 3−1475) and IRAS 17441−2411, is presented. H2 line emission is detected in discrete shock fronts in the lobes of each nebula, regardless of source spectral type[although non-detections in IRAS 09371+1212 (Frosty Leo) support claims that late spectral types do not produce bright H2 line emission]. In IRAS 17150−3224, we also uncover possible signs of rotation, as would be expected if the H2 features were excited in a magnetocentrifugal disc wind.[Fe ii] 1.644 μm emission was detected in only one source, M 1−92 (notably, the source with the brightest H2 features). Again, the emission is predominantly excited in high-velocity shocks in the bipolar lobes of the PPN. The H2 and[Fe ii] observations of M 1−92, and the complex H2 profiles in IRAS 21282+5050, are explained using the shock models of Smith and collaborators. We show that bow shock models are generally able to account for the observed line profiles, peak velocities, the double-peaked profiles in IRAS 21282+5050, and the spatial distribution of H2 and[Fe ii] in M 1−92. J-type bow models are adequate in each case, i.e. a strongly magnetized wind is not required.Finally, Brγ is detected in each of the five targets; in absorption in the G-type PPN, though in emission in the O- and B-type sources. Brγ emission is detected predominantly towards the near-infrared (near-IR) continuum peak in each PPN, with only very weak emission detected in the extended lobes of the O- and B-type sources. In Brγ, low peak velocities, though very broad profile widths, are measured in each target, regardless of the nebula inclination angle. The emission must therefore derive from ionized regions in a fast wind very close to the central star (rather than from shocks in the bipolar lobes), or, in the late-type sources, from absorption in an equatorial torus. [ABSTRACT FROM AUTHOR]

We present a study of calibrated low-resolution spectra of the 18 known primaries of the≈9-Myr-oldη Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravity-sensitive indices derived from the spectra, we compare theη Cha stars to standard dwarfs. We find that theVRIcolours of the PMS stars are indistinguishable from those of main-sequence stars, but that aB-band excess attaining≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for theη Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung–Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-typeη Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that theη Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirmmag likely due to the presence of circumstellar material. [ABSTRACT FROM AUTHOR]

We present synthetic spectra for circumstellar discs that are heated by radiation from a central brown dwarf. Under the assumption of vertical hydrostatic equilibrium, our models yield scaleheights for brown dwarf discs in excess of three times those derived for classical T Tauri (CTTS) discs. If the near-IR excess emission observed from brown dwarfs is indeed due to circumstellar discs, then the large scaleheights we find could have a significant impact on the optical and near-IR detectability of such systems. Our radiation transfer calculations show that such highly flared discs around brown dwarfs will result in a large fraction of obscured sources due to extinction of direct starlight by the disc over a wide range of sightlines. The obscured fraction for a ‘typical’ CTTS is less than 20 per cent. We show that the obscured fraction for brown dwarfs may be double that for CTTS, but this depends on stellar and disc mass. We also comment on possible confusion in identifying brown dwarfs via colour–magnitude diagrams: edge-on CTTS display similar colours and magnitudes to face-on brown dwarf plus disc systems. [ABSTRACT FROM AUTHOR]

ABSTRACT We have observed 22 young stars from the Lindroos sample at 850 μm with SCUBA on the James Clerk Maxwell Telescope to search for evidence of dust discs. Stars in this sample are the less massive companions of B-type primaries and have well-defined ages that are 10– 170 Myr; i.e. they are about to, or have recently arrived on the main sequence. Dust was detected around three of these stars (HD 112412, 74067 and 99803B). The emission around HD 74067 is centrally peaked and is approximately symmetrically distributed out to ∼70 arcsec from the star. This emission arises from either a two-component disc, one circumstellar and the other circumbinary with dust masses of 0.3 and <27 M[sub ⊕], respectively, or an unrelated background object. The other two detections we attribute to circumsecondary discs with masses of 0.04 and 0.3 M[sub ⊕]. We were also able to show that a circumprimary disc is present around HD 112413 with a similar mass to that around the companion HD 112412. Cross-correlation of our sample with the IRAS catalogues only showed evidence for dust emission at 25 and 60 μm toward one star (HD 1438); none of the submillimetre detections were evident in the far-infrared data implying that these discs are cold (>40 K assuming β= 1). Our submillimetre detections are some of the first of dust discs surrounding evolved stars that were not detected by IRAS or ISO and imply that 9–14 per cent of stars could harbour previously undetected dust discs that await discovery in unbiased sub-mm surveys. If these discs are protoplanetary remnants, rather than secondary debris discs, dust lifetime arguments show that they must be devoid of small <0.1 mm grains. Thus it may be possible to determine the origin of these discs from their spectral energy distributions. The low inferred dust masses for this sample support the picture that protoplanetary dust discs are depleted to the levels of the brightest debris discs (∼1 M[sub ⊕]) within 10 Myr, although if the extended emission of HD 74067 is associated with the star, this would indicate that >10 M[sub ⊕] of circumbinary material can persist until ∼60 Myr and would also support the theory that T Tauri discs in binary systems are replenished by circumbinary envelopes. [ABSTRACT FROM AUTHOR]

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be &lsim;1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities (L[sub ir]/L[sub ★]) and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low L[sub ir]/L[sub ★], the scarcity of circumstellar dust, the low polarization, the presence of H α emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars. [ABSTRACT FROM AUTHOR]

ABSTRACTWe present high-resolution spectroscopy of the variable Ca II H & K absorptions in the star β Pictoris. 313 spectra were obtained on 53 nights between 1994 November and 1996 December in order to characterize the behaviour of the lines and to test the falling evaporating bodies (FEB) scenario. Near-continuous absorption activity was seen, including blueshifted features, one of which rivalled the strength of that reported in 1997 June by Crawford, Beust & Lagrange. Redshifted features at ∼ 5--20 km s-1 were present until the end of 1995, and are evocative of a year-long stream of infalling bodies with typically 4--5 objects in the line of sight. At higher velocities, features were shorter lived and generally broader and shallower. These correlations also apply to blueshifted features. On 1995 June 9 we detected a probable narrow, short-lived feature at a redshift of ∼ 130 km s-1. The FEB models predict that the strength of the H line may exceed the K line value as a body tracks across the stellar disc, but we have no convincing observation of this, although we did observe some events during which the H & K absorptions evolved differently. [ABSTRACT FROM AUTHOR]