Atmospheric analysis of the M/L and M/T dwarf binary systems LHS 102 and Gliese 229

We present  0.9–2.5 μm  spectroscopy with   R ∼800  and  1.12–1.22 μm  spectroscopy with   R ∼5800  for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS 102B. We also report IZJHKL ' photometry for both components of the LHS 102 system, and L ' photometry for Gl 229A. The data are combined with previously published spectroscopy and photometry to produce flux distributions for each component of the kinematically old disc M/L dwarf binary system LHS 102 and the kinematically young disc M/T dwarf binary system Gliese 229. The data are analysed using synthetic spectra generated by the latest 'AMES-dusty' and 'AMES-cond' models by Allard & Hauschildt. Although the models are not able to reproduce the overall slope of the infrared flux distribution of the L dwarf, most likely because of the treatment of dust in the photosphere, the data for the M dwarfs and the T dwarf are well matched. We find that the Gl 229 system is metal-poor despite having kinematics of the young disc, and that the LHS 102 system has solar metallicity. The observed luminosities and derived temperatures and gravities are consistent with evolutionary model predictions if the Gl 229 system is very young  (age∼30 Myr)  with masses (A,B) of (0.38,≳0.007) M_⊙, and the LHS 102 system is older, aged  1–10 Gyr  with masses (A,B) of (0.19,0.07) M_⊙.     

Possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm

We report the possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm with SCUBA on the James Clerk Maxwell Telescope. The submillimetre photometry, combined with a  1–5 μm  spectrum and  8–10 μm  photometry obtained nearly contemporaneously, suggests that the submillimetre emission originates in material ejected during the 1995 event. The dust mass is a  few×10^-7 M_⊙  , the average mass-loss in the form of dust is  few×10^-8 M_⊙ yr^-1  , and the integrated luminosity is  log( L /L_⊙)=3.66  for a distance of 2 kpc. The ejected shell had angular diameter ∼55 mas in 2001 August, and should by now be resolvable in the mid-infrared by  8–10 m  class telescopes.     

An attempt to identify the pulsation modes in the δ Scuti star QQ Tel

We present simultaneous UBVRI photometry and high-dispersion spectroscopy of the δ Scuti star QQ Tel. At least seven periodicities are detected in the light curve, but there are likely to be many more. The line profile variations suggest that some of the observed frequencies may be due to modes of moderately high degree  (ℓ≳4)  .     

Mode identification in the δ Scuti Star 1 Mon

We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree,     the azimuthal order, m , of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of     for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably         We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the     mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star.     

Optical studies of two Large Magellanic Cloud X-ray transients: RX J0544.1–7100 and RX J0520.5–6932

We report observations that confirm the identities of the optical counterparts to the transient sources RX J0544.1–7100 and RX J0520.5–6932 . The counterparts are suggested to be B-type stars. Optical data from the observations carried out at ESO and SAAO, together with results from the OGLE data base, are presented. In addition, X-ray data from the RXTE All-Sky Monitor are investigated for long-term periodicities. A strong suggestion for a binary period of 24.4 d is seen in RX J0520.5–6932 from the OGLE data.     

The distribution of bright OB stars in the Canis Major–Puppis–Vela region of the Milky Way

The picture of the young stellar groups in the Canis Major–Puppis–Vela (215°< l <275°) section of the Milky Way is studied and updated utilizing uvbyβ photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag.
The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (±0.05 s.e. ) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5–4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (±0.14 s.e.) kpc – much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (±0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range.
In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20–25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.     

The EC 14026 stars – VI. PG 1047+003

We report the discovery of rapid oscillations in the sdB star PG 1047+003. The oscillations are multiperiodic, with nine periods in the range 104–162 s. The optical spectrum of PG 1047+003 is consistent with that of a single sdB star. Line profile fitting yields an effective temperature of 35 000±1000 K and log g =5.9±0.1, although optical photometry and IUE spectrophotometry may indicate a cooler effective temperature. These properties demonstrate that PG 1047+003 is an EC 14026 star, a recently discovered class of sdB pulsators. Optical and infrared photometry constrains any cool companion to the sdB star to be a main-sequence star of spectral type M0 or later. With V =13.47 and a relatively rich pulsation spectrum, PG 1047+003 is an attractive target for an intensive photometric campaign to extract more periods from the light curve which, along with a suitable grid of pulsation models, will probe the interior structure of the star.     

uvbyβ photometry of the short-period binary VV Ursae Majoris

We present well-sampled uvby light curves, supplemented by a few β filter measurements, of the Algol binary VV UMa. The light curves are analysed using two different codes to derive the orbital and absolute stellar parameters of this binary. We find reasonably good fits to the light curves and determine the stellar effective temperatures T _eff,1≃9000–9600 K , and T _eff,2≃5300–5600 K with a mass ratio q ≃0.35 . From the light-curve fits we discard the possibility of an anomalous gravity-darkening exponent for the secondary star of this system, as previously suggested.
We find evidence of short-term, small-amplitude variations in the brightness of the system. Two periodicities of about 1.10 and 0.51 h seem to be present in the data for at least two different nights, even within the secondary eclipse. This suggests that VV UMa may be a new Algol binary with a low-amplitude variable primary star, but new data collected during longer observing runs are necessary to confirm the pulsating nature of the brightness variations.     

Resolving the pulsations of the subdwarf B star KPD 2109 + 4401

We present the results of extensive time-series photometry of the pulsating subdwarf B star KPD 2109 + 4401. Our data set consists of 29 data runs with a total length of 182.6 h over 31 d, collected at five observatories in 2004. These data comprised high signal-to-noise ratio observations acquired with larger telescopes and wider time-coverage observations obtained with smaller telescopes. They are sufficient to resolve the pulsation structure to 0.4 μHz and are the most extensive data set for this star to date. With these data, we identify eight pulsation frequencies extending from 4701 to 5481 μHz, corresponding to periods of 182–213 s. The pulsation frequencies and their amplitudes are examined over several time-scales with some frequencies showing amplitude variability.     

Spectroscopic observations of the eclipsing polar MN Hya (RX J0929–24)

We present low–medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929–24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42 MG from the spacing of cyclotron features visible during π ∼ 0.4–0.7. From spectra taken during the eclipse we find that the secondary has an M3–4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada &38; Bassett and an estimate of the interstellar extinction we find a distance of ∼300–700 pc. We find unusual line variations at π ∼ 0.9: Hα is seen in absorption and emission. This is at the same point in the orbital phase at which a prominent absorption dip is seen in soft X-rays.     

KPD 0629–0016, a slowly pulsating hot subdwarf star

The results of nine CCD photometric observing runs on KPD 0629–0016 are presented. During six of the runs measurements were obtained alternately through B and V filters. Four periodicities, for which there is good agreement between the frequencies identified in the B and V data, were extracted: these lie in the range 46–81 min. A fifth lower frequency appears to be definitely present, but is very uncertain due to aliasing. The ratios of the mode amplitudes measured in B and V , and the phase differences between variations in the two colours, are compatible with pulsation theory.     

Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σ Ori cluster

The properties of accretion discs around stars and brown dwarfs in the σ Ori cluster (age 3 Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ∼30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J -band amplitudes  ≥0.5 mag  ; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of  ∼0.5–8 d  , but have additional irregular variations superimposed – the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000–7000 K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer , we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σ Ori cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.     

Discovery and asteroseismological analysis of a new pulsating DB white dwarf star, PG 2246+121

We report 36.6 h of time-resolved CCD photometry of the DB white dwarf star PG 2246+121 and the discovery that it is a new pulsating variable. Analysis of our compact single-site data set allowed the detection of three mode multiplets, two triplets at 256 and 329 s, respectively, and one doublet at 286 s. The frequency splitting within those structures is exactly the same within the length and accuracy of our data set.
We argue that these multiplets are the result of non-radial g-mode pulsations, most probably of spherical degree ℓ=1, which then yields a formal stellar rotation period of 2.00±0.12 d. We suggest that the excited modes are three consecutive radial overtones of order 3–7, most likely k =4,5,6. This discovery's impact on the understanding of pulsating DB white dwarfs is discussed.     

Far-red optical colours of late-M and L dwarfs

We present both synthetic and observed   I - Z   colours for a sample of spectroscopically classified M and L dwarfs drawn primarily from recent wide-field infrared surveys. Our synthetic photometry does not reproduce the strong decrease in the   I _Harris- Z _RGO  natural colour beyond M9 to a minimum around L2.5 reported by Steele & Howells. However, it does support the result that the   I _Harris- Z _RGO  colour does not increase with spectral type between  L0–L2.5  . Instead, the   I _Harris- Z _RGO  ,   i _WFC- Z _WFC  ,   i _SDSS- z _SDSS  and   I _C- Z _RGO  colours are shown to remain more or less constant before increasing once again for types later than  L3–L4  . These results, which are supported by both previously published observations and new photometry obtained with the Isaac Newton Telescope (INT) and the Wide Field Camera (WFC), have implications for the interpretation of the recently publicly released Wide Field Survey Programme data covering the Pleiades cluster and surveys for late-type M and L dwarfs in general.     

Rotation and lithium in the late-type stars of NGC 2516

We present CCD photometry and high-resolution spectroscopy of low-mass stars in the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the Pleiades. 24 probable F to early K type, single cluster members have been identified from their photometry and radial velocities, along with three possible spectroscopic binaries. The projected equatorial velocities are measured and compared with younger and older clusters. Several fast rotating late G /early K stars are seen, but all hotter stars have v _e sin  i  < 20 km s⁻¹. The data are consistent with angular momentum loss models with spin-down time-scales that increase from tens of Myr for G stars to hundreds of Myr for K stars. The observed X-ray activity is consistent with the currently accepted rotation–activity paradigm. Lithium abundances are derived from the Li  i 6708-Å line. The pattern of Li depletion is indistinguishable from that in the Pleiades, including a spread in the K0 stars, where the most rapid rotators suffer the least Li depletion. The observations argue in favour of either a metallicity in the range −0.1 < [Fe/H]< 0.0 for NGC 2516, or a lower metallicity and extra Li depletion through non-standard mixing modes which occurs on time-scales of only ∼ 50 Myr. Neither our low signal-to-noise ratio spectroscopy nor our photometry can constrain [Fe/H] sufficiently to decide between these possibilities. A detailed spectroscopic chemical abundance analysis is urgently required.     

Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in red giant stars

Measuring solar-like oscillations in an ensemble of stars in a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field stars. The most-ambitious attempt to pursue these prospects was by Gilliland et al. who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (<20 μmag) to obtain unambiguous detections. Like Gilliland et al. we also aim at detecting solar-like oscillations in M67, but we target red giant stars with expected amplitudes in the range 50–  500 μmag  and periods of 1 to 8 h. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compared with simulations and with estimated properties of the stellar oscillations. Noise levels in the Fourier spectra as low as  27 μmag  are obtained for single sites, while the combined data reach  19 μmag  , making this the best photometric time series of an ensemble of red giant stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.     

Multiperiodic variations in the last 104-yr light curve of the symbiotic star BF Cyg

We analyse a light curve (LC) of the symbiotic star BF Cyg, covering 114 yr of its photometric history. The star had a major outburst around the year 1894. Since then the mean optical brightness of the system is in steady decline, reaching only in the last few years its pre-outburst value. Superposed on this general decline are some six less intense outbursts of 1–2 mag and duration of 2000–5000 d. We find a cycle of 6376 d, or possibly twice this period, in the occurrence of these outbursts. We suggest that the origin of the system outbursts is in some magnetic cycle in the outer layers of the giant star of the system, akin to the less intense 8000-d magnetic cycle of our Sun. We further find, that in addition to its well-known binary period of 757.3 d, BF Cyg possesses also another photometric period of 798.8 d. This could be the rotation period of the giant star of the system. If it is, the beat period of these two periodicities, 14 580 d, is the rotation period of a tidal wave on the surface of the giant. A fourth period of 4436 d, the beat period of the 14 580-d and the 6376-d cycles is possibly also present in the LC. We predict that BF Cyg will be at the peak of its next outburst around the month of May in the year 2007. The newly discovered 798.8-d period explains the disappearance of the orbital modulation at some epochs in the LC. The 757.3-d oscillations will be damped again around the year 2013.     

The domain of γ Doradus variables in the Hertzsprung–Russell diagram

70 new γ Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Selecting the objects with good evidence for multiperiodicity, it is found that these stars, together with bona fide members of the class, occupy a well-defined region in a colour–magnitude diagram. This domain corresponds to a range of 7200–7700 K on the zero-age main sequence (ZAMS) and 6900–7500 K one magnitude above it, which partly overlaps with the instability strip of δ Scuti stars.
For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed.     

Photometric distances to nine dark globules

Distances to nine dark globules are determined by a method using optical ( VRI ) and near-infrared (near-IR) ( JHK ) photometry of stars projected towards the field containing the globules. In this method, we compute intrinsic colour indices of stars projected towards the direction of the globule by dereddening the observed colour indices using various trial values of extinction   A _V   and a standard extinction law. These computed intrinsic colour indices for each star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match with the standard intrinsic colour indices. Distances ( d ) to the stars are determined using the   A _V   and absolute magnitude  ( M_V )  corresponding to the spectral types thus obtained. A distance versus extinction plot is made and the distance at which   A _V   undergoes a sharp rise is taken to be the distance to the globule. All the clouds studied in this work are in the distance range 160–400 pc. The estimated distances to dark globules LDN 544, LDN 549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN 1257 are  180 ± 35, 200 ± 40, 180 ± 35, 160 ± 30, 350 ± 70, 200 ± 40, 400 ± 80, 250 ± 50  and 250 ± 50 pc, respectively. Using the distances determined, we have estimated the masses of the globules and the far-IR luminosity of the IRAS sources associated with them. The mass of the clouds studied are in the range  10–200 M_⊙  .     

Radial velocity variations of the pulsating subdwarf B star PG 1605+072

We present an analysis of high-speed spectroscopy of the pulsating subdwarf B star PG 1605+072. Periodic radial motions are detected at frequencies similar to those reported for photometric variations in the star, with amplitudes of up to 6 km s⁻¹. Differences between relative strengths for given frequency peaks for our velocity data and previously measured photometry are probably a result of shifting of power between modes over time. Small differences in the detected frequencies may also indicate mode-shifting. We report the detection of line-shape variations using the moments of the cross-correlation function profiles. It may be possible to use the moments to identify the pulsation modes of the star.