Rapid oscillations of five stars in the spectral range G and K. II

The five stars HD 11112, HD 170747, HD 186651, HD 205390 and HD 224834 are a second group of objects enclosed in a larger observational programme for the search of rapid oscillations. They have been observed with the 50 cm photoelectrical ESO telescope at La Silla (Chile). For the star HD 205390 a variation of 163.8 min was found. The two objects HD 11112 and HD 186651 are suspected to be variable with periods of 139 min and 142.2 min and its overtones, respectively. But a verification of these results is necessary. For the last two objects HD 170747 and HD 224834 no variations were detected.     

Eight new δ Scuti stars

HD 23194, a member of the Pleiades, was found to pulsate with a period of about 30 min. The literature on the star is reviewed, and it is concluded that it may be a marginal Am star in a binary system. HD 95321 is an evolved Am ( ρ Puppis) star with a 5.1-h periodicity. Mode identification of its pulsation, based on multicolour photometry, suggests that the oscillation is probably non-radial with ℓ=2. We also report on the discovery of six other new δ Scuti stars, some of which may be pulsating in gravity modes.     

HD 12098 and other results from Nainital-Cape Survey

Nainital-Cape Survey was started with an aim to search for new rapidly oscillating Ap stars in the northern hemisphere. We discovered one new mono-periodic roAp star HD 12098. The frequency separation of HD 12098 suggests a rotation period of 5.5 days for the star. We summarize here the observations of HD 12098 and briefly discuss the results of the multi-site observation campaign organized to resolve the ambiguity in the determination of the rotation period of HD 12098. Other interesting results like non-oscillating Ap stars discovered and two candidate stars in which roAp periodicity is seen but not confirmed are also discussed.     

Pulsation in the pre-main-sequence Herbig Ae star HD 142666

Asteroseismology of pre-main-sequence δ Scuti stars has the potential not only to provide unprecedented constraints on models of these stars, but also to allow for the possibility of detecting evolutionary period changes, thus providing a direct measure of the pre-main-sequence evolutionary time-scale. In the last two years, the published number of such stars known has doubled from four to eight. Searches are now being conducted amongst the Herbig Ae stars, which are considered to be excellent candidates. We announce the discovery of δ Scuti pulsation in one Herbig Ae star, HD 142666, which lies within Marconi & Palla's theoretically predicted instability strip for pre-main-sequence stars, making this the ninth known pre-main-sequence δ Scuti star. We also demonstrate a lack of δ Scuti pulsation in another such star, HD 142527.     

Spectroscopic binaries near the north Galactic pole paper 24: HD 106104, 109281, 109463 and 110743

The four stars treated in this paper have been under observation with photoelectric radial-velocity spectrometers for many years. They have proved to be binaries with periods of 30, 1828, 1514 and 822 days respectively; the orbits are of modest eccentricity apart from that of HD 110743 which is indistinguishable from a circle. The mass functions are small, and no companion has been observed for any of the stars. HD 110743, a K dwarf, is much the nearest of the four, and its orbit is of short enough period for the photocentric motion to have been recognized byHipparcos. An eleventh-magnitude star rather more than 1≈ away from HD 106104 is shown to be a genuine physical companion, with practically identical radial velocity, proper motion and distance modulus, although the projected separation is about 13,000 AU.     

Discovery of magnetic fields in three He variable Bp stars with He and Si spots

It is essential for the understanding of stellar structure models of high mass stars to explain why constant stars, nonpulsating chemically peculiar hot Bp stars and pulsating stars co‐exist in the slowly pulsating B stars and β Cephei instability strips. We have conducted a search for magnetic fields in the four Bp stars HD55522, HD105382, HD131120, and HD138769 which previously have been wrongly identified as slowly pulsating B stars. A recent study of these stars using the Doppler Imaging technique revealed that the elements He and Si are inhomogeneously distributed on the stellar surface, causing the periodic variability. Using FORS 1 in spectropolarimetric mode at the VLT, we have acquired circular polarisation spectra to test the presence of a magnetic field in these stars. A variable magnetic field is clearly detected in HD55522 and HD105382, but no evidence for the existence of a magnetic field was found in HD131120. The presence of a magnetic field in HD138769 is suggested by one measurement at 3σ level. We discuss the occurrence of magnetic B stars among the confirmed pulsating B stars and find strong magnetic fields of order kG and oscillations to be mutually exclusive. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the class of Oe stars'

We present high‐quality spectra of the majority of stars that have been classified as Oe and find that their published spectral types are generally too early, most likely due to infilling of He I lines. As a matter of fact, all stars classified as Oe actually fall inside the range O9–B0 with the important exception of HD 155806 (O7.5 III) and perhaps HD 39680 (difficult to classify, but likely O8.5V). Observations of a sample of objects with published spectral types in the O9–B0 range previously classified as peculiar or emission‐line stars fail to reveal any new Oe star with spectral type earlier than O9.5. Most objects classified as peculiar in “classical” literature show signs of binarity in our spectra, but no spectral anomalies. We conclude that there is likely a real decline in the fraction of Be stars for spectral types earlier than B0, not due to observational bias. The few Oe stars with spectral types earlier than O9.5 deserve detailed investigation in order to provide constraints on the physical reasons of the Be phenomenon. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A search for luminous Be stars

As Be stars are restricted to luminosity classes III‐V, but early B‐type stars are believed to evolve into supergiants, it is to be expected that the Be phenomenon disappears at some point in the evolution of a moderately massive star, before it reaches the supergiant phase. As a first stage in an attempt to determine the physical reasons of this cessation, a search of the literature has provided a number of candidates to be Be stars with luminosity classes Ib or II. Spectroscopy has been obtained for candidates in a number of open clusters and associations, as well as several other bright stars in those clusters. Among the objects observed, HD 207329 is the best candidate to be a high‐luminosity Be star, as it appears like a fast‐rotating supergiant with double‐peaked emission lines. The lines of HD 229059, in Berkeley 87, also appear morphologically similar to those of Be stars, but there are reasons to suspect that this object is an interacting binary. At slightly lower luminosities, LS I +56°92 (B4 II) and HD 333452 (O9 II), also appear as intrinsically luminous Be stars. Two Be stars in NGC 6913, HD 229221 and HD 229239, appear to have rather higher intrinsic magnitudes than their spectral type (B0.2 III in both cases) would indicate, being as luminous as luminosity class II objects in the same cluster. HD 344863, in NGC 6823, is also a rather early Be star of moderately high luminosity. The search shows that, though high‐luminosity Be stars do exist, they are scarce and, perhaps surprisingly, tend to have early spectral types. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anomalous abundance of lithium in the roAp star HD 12098 recently discovered in the northern hemisphere of the sky

Spectral observations of Ap-CP stars with the BTA (Special Astronomical Observatory, Russian Academy of Sciences) using the NES echelle spectrometer have revealed several stars with an anomalous lithium abundance. The oscillating star HD 12098, which is the first roAp star in the northern hemisphere of the sky, merits special attention. Strong, variable LiI 6708 Å line was observed in the spectrum of this star. There are not enough observations for a reliable analysis by Doppler mapping, but there are enough to indicate the presence of lithium spots on the surface of this star similar to the roAp stars HD 83368 and HD 60435, on whose surfaces spots with a high lithium abundance have been reliably detected. Parameters for a model of its atmosphere have been chosen using the method of synthetic spectra based on atmospheric models including lines from the VALD list and several additional blended REE lines calculated by the authors. The profile of the lithium LiI 6708 Å blend has been calculated taking the magnetic field into account using the SYNTHM code. A lithium abundance has been determined for two phases of the rotation of HD 12098 that is anomalously high compared to the solar and meteoritic abundances. The large difference in the lithium abundance in the two phases (in two different regions on the star's surface) exceeds 0.5 dex and is very close to that which we have found by analyzing the spectra of the roAp stars HD 83368 and HD 60435. Thus, we have discovered yet another roAp star, HD 12098, with lithium spots on its surface. Translated from Astrofizika, Vol. 51, No. 4, pp. 607–616 (November 2008).     

Abundance analysis of the γ Doradus–δ Scuti hybrid metallic line (Am) star HD 8801

Low frequency oscillation, typical for γ Doradus g‐mode type stellar core sensitive pulsation, as well as higher frequency δ Scuti type pulsation typical for p ‐modes, sensitive to the envelope, make HD 8801 a remarkable hybrid pulsator with the potential to probe a stellar structure over a wide range of radius. In addition HD 8801 is a rare pulsating metallic line (Am) star. We determined the astro‐physical fundamental parameters to locate HD 8801 in the H‐R diagram. We analyzed the element abundances, paying close attention to the errors involved, and confirm the nature of HD 8801 as a metallic line (Am) star. We also determined an upper limit on the magnetic field strength. Our abundance analysis is based on classical techniques, but uses for the final step a model atmosphere calculated with the abundances determined by us. We also discuss spectropolarimetric observations obtained for HD 8801. This object is remarkable in several respects. It is a nonmagnetic metallic line (Am) star, pulsating simultaneously in p‐ and g‐modes, but also shows oscillations with periods in between these two domains, whose excitation requires explanation. Overall, the pulsational incidence in unevolved classical Am stars is believed to be quite low; HD 8801 does not conform to this picture. Finally, about 75 % of Am stars are located in short‐period binaries, but there is no evidence that HD 8801 has a companion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Non-emission-line young stars of intermediate mass

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 ≲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 _ir/ L _★)  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 _ir/ L _★  , 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.     

Optical, X-ray and radio observations of HD 61396: a probable new RS CVn-type binary

We have carried out BVR photometric and H spectroscopic observations of the star HD 61396 during 1998 March 20 to 1999 April 3. We have discovered regular optical photometric variability from this star, with an inferred period of 31.95±0.10 d, and an amplitude of 0.18 mag. A possible period of 35.34±0.12 d, as determined with Hipparcos , cannot be completely ruled out, however. Modelling of its photometric light curve with two circular spots indicates that 521 per cent of the stellar surface is covered by dark starspots which are 830 K cooler than the surrounding photosphere, and produce the observed rotational modulation of the optical flux. Optical spectroscopy reveals a variable H emission feature, indicating that it is an unusually active star.
In addition, we have analysed archival X-ray data of HD 61396, obtained from serendipitous observations with the ROSAT X-ray observatory, and we also discuss the radio properties of this star, based on both published Green Bank and unpublished VLA observations. The strong photometric variability and H emission, the relatively hard X-ray spectrum, and the high X-ray and radio luminosities imply that HD 61396 is most likely to be a member of the RS CVn class of evolved active binary stars. Its X-ray and radio luminosities place it among the five most luminous active binaries detected so far.     

The evolved B[e] star HD 87643: observations and a radiation-driven disc wind model for B[e] stars

New high-resolution spectroscopic and medium-resolution spectropolarimetric data of the B[e] star HD 87643 are presented, complemented with optical broad- and narrow-band imaging. The spectrum of HD 87643 exhibits the hybrid characteristics well known to be representative of the group of B[e] stars; a fast wind with an expansion velocity in excess of 1000 km s⁻¹ is measured in the hydrogen and helium lines, while a slower component is traced by lower excitation lines and forbidden lines. Clues to the geometry of the rapidly expanding circumstellar shell are provided by the startling polarization changes across Hα. Comparison with published schematic calculations indicates that the polarizing material is located in a slowly rotating, expanding disc structure. A hydrodynamical model is then presented, the results of which are consistent with the original two-wind concept for B[e] stars, and which exhibits kinematic properties that may well explain the observed spectral features in HD 87643. The model calculations use as input a B star undergoing mass loss, surrounded by an optically thick disc. The resulting configuration consists of a fast polar wind from the star and a slowly expanding disc wind. The model also predicts that the stellar wind at intermediate latitudes is slower and denser than in the polar region.     

Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold

From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.
We reconstruct the large-scale magnetic geometry of the targets as a low-order  (ℓ < 10)  spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component.     

HD 115781 and HD 116204—two RS CVn binaries

HD 115781 and HD 116204 (BL CVn and BM CVn) are shown to be RS CVn binaries with periods near 20 days. HD 115781 is double-lined; the primary type is about K1III, while the secondary is probably a late-type subgiant. The masses of the two components are equal within observational error. There is substantial photometric variability with a period half the orbital period; it is attributed to ellipsoidal variation. HD 116204 is also of type K1III. It shows exceptionally strong Ca II H and K emission, together with an emission-line spectrum typical of RS CVn stars in theIUE ultraviolet region, but Hα is an absorption line. The secondary star in the HD 116204 system has not been detected. The primary shows photometric variations, presumably due to starspots, with a period 5 per cent longer than the orbital period.     

UBVRIJH photometry of two new luminous δ Scuti stars and the discovery of δ Scuti pulsation in the most evolved Ap star known

Time-series photometry of the Hipparcos variable stars HD 199434 and 21190 is reported. Both stars are pulsators of the δ Scuti type. Reclassifications of the MK types of the stars, based on new spectrograms, are given. HD 21190 is found to be F2III SrEuSi:, making it the most evolved Ap star known. Its Strömgren photometric indices support the peculiar spectral type. It is also one of the most evolved δ Scuti stars known. Its combined Ap– δ Scuti nature makes it an important test of models of pulsation in peculiar stars recently developed by Turcotte et al., although it is more extreme than any model they examined. Physical parameters of both stars are estimated from Strömgren and H β photometry, and Hipparcos absolute magnitudes. We attempt mode identifications based on amplitude ratios and phase differences from our photometry. The dominant pulsation of HD 21190 may be an overtone radial mode. The model fits for HD 199434 are even less satisfactory, but favour an ℓ=2 mode. Given the good quality and wavelength coverage of our data, the poor results from the application of the photometric theory of mode identification may call into question the use of that technique.     

The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The evolutionary state of magnetic Ap stars

The characteristics of two stars, 25 Sex and HD 21699, as additional candidates for the sample of magnetic stars belonging to superclusters, are discussed. For 25 Sex, which was already accepted as a probable member of the Hyades supercluster in a previous study, arguments supporting the view that this star indeed is a magnetic star are presented. In the case of HD 21699, the radial velocity derived from our observations is not inconsistent with membership. But from the determinations of its proper motion found in the literature, this star cannot be regarded as a probable member of the α Per cluster. On the basis of recent evolutionary models, all the well established Ap cluster members appear to be close to the end of their main-sequence life. This suggests that A stars possibly become magnetic at the end of core hydrogen burning.     

Contribution to the study of F‐G‐K‐M binaries: XII. Orbital elements of seven new spectroscopic binaries

The orbital elements of seven single‐lined spectroscopic binaries, HD 31855, HD 59643, HD 60092, HD 133189, HD 162262, HD 203522, BD +43° 1331 are presented. They are obtained from observations made with two photoelectric spectrometers of CORAVEL type, the first located at the Observatoire de Haute‐Provence and the second at the Cambridge Observatories. From those orbital elements and other data available in the literature, we deduce some information about the unseen companions and their separations with respect to the primaries. We then discuss the rotation‐revolution synchronism and conclude that some of those stars have probably reached the state of (pseudo‐)synchronism. Finally we give a synopsis of the results obtained in this series of papers. Our radial velocity monitoring of more than three decades has allowed us to derive accurate orbital elements of 35 spectroscopic binaries, with a cool primary star of type F‐G‐K‐M. The corresponding range of periods varies from a few days to more than 10 years. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurements of mean longitudinal magnetic fields in the Of?p stars HD 108 and HD 191612

Using polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope, we detect a longitudinal magnetic field 〈B_z〉 = –168±35 G in the Of?p star HD 108. This result is in agreement with the longitudinal magnetic field measurement of the order of –150 G recently reported by the MiMeS team. The measurement of the longitudinal magnetic field in the Of?p star HD 191612 results in 〈B_z〉 = +450±153 G. The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field 〈B_z〉 = –220±38 G, indicating a change of polarity over ∼100 days. Further, we report the detection of distinct Zeeman features in the narrow Ca II and Na I doublet lines for both Of?p stars, hinting at the possible presence of material around these stars. The origin of these features is not yet clear and more work is needed to investigate how magnetic fields interact with stellar wind dynamics (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)