Early-type Main-sequence Stars Research Articles

A substellar companion around the intermediate-mass giant star HD 11977

We report the discovery of a substellar companion to the intermediate-mass star HD 11977 (G5 III). Radial velocities of this star have been monitored for five years with FEROS at the 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in of K 1 = Chile. Based on the collected data we calculated an orbital solution with a period of P = 711 days, a semi-amplitude of K 1 = 105 ms -1 , and an eccentricity of e = 0.4. The period of the radial-velocity variation is longer than that of the estimated stellar rotation, rendering it unlikely that rotational modulation is the source of the variation in the radial velocity. This hypothesis is supported by the absence of a correlation between stellar activity indicators and radial-velocity variation. By determining a primary stellar mass of M * = 1.91 M ○. , the best-fit minimum mass of the companion and semi-major axis of the orbit are m 2 sin i = 6.54 M Jup and a 2 = 1.93 AU, respectively. An upper limit for the mass of the companion of m 2 ? 65.5 M Jup has been calculated from HIPPARCOS astrometric measurements. Although the possibility of a brown-dwarf companion cannot be excluded, HD 11977 B is one of the few planet candidates detected around an intermediate-mass star. The progenitor main-sequence star of HD 11977 is probably an A-type star. This discovery gives an indirect evidence for planetary companions around early type main-sequence stars.

Looking Deep Within an A-type Star: Core Convection Under the Influence of Rotation

The advent of massively parallel supercomputing has begun to permit explicit 3–D simulations of turbulent convection occurring within the cores of early-type main sequence stars. Such studies should complement the stellar structure and evolution efforts that have so far largely employed 1–D nonlocal mixing length descriptions for the transport, mixing and overshooting achieved by core convection. We have turned to A-type stars as representative of many of the dynamical challenges raised by core convection within rotating stars. The differential rotation and meridional circulations achieved deep within the star by the convection, the likelihood of sustained magnetic dynamo action there, and the bringing of fresh fuel into the core by overshooting motions, thereby influencing main sequence lifetimes, all constitute interesting dynamical questions that require detailed modelling of global-scale convection. Using our anelastic spherical harmonic (ASH) code tested on the solar differential rotation problem, we have conducted a series of 3–D spherical domain simulations that deal with a simplified description of the central regions of rotating A-type stars, i.e a convectively unstable core is surrounded by a stable radiative envelope. A sequence of 3–D simulations are used to assess the properties of the convection (its global patterns, differential rotation, meridional circulations, extent and latitudinal variation of the overshooting) as transitions are made between laminar and turbulent states by changing the effective diffusivities, rotation rates, and subadiabaticity of the radiative exterior. We report on the properties deduced from these models for both the extent of penetration and the profile of rotation sustained by the convection.

Rotational mixing in early-type main-sequence stars

We present quantitative observational investigations into the importance of rotationally induced mixing in late-O stars. First, we conduct non-LTE, hydrostatic, plane-parallel H/He model-atmosphere analyses of the optical spectra of three of the most rapidly rotating late-O near-main-sequence stars known: HD 93521 (O9.5 V), HD 149757 (ζ Ophiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which have equatorial rotation velocities of ∼ 430 km s−1 and . The analysis allows for the expected (von Zeipel) variation of Teff and log g with latitude. These three stars are found to share very similar characteristics, including substantially enhanced surface-helium abundances . Secondly, we compare the distribution of projected rotational velocities for ON and morphologically normal dwarf O stars, and demonstrate that the ON stars are drawn from a population with more rapid rotation. The results provide qualitative support for rotationally induced mixing, although there remain discrepancies between atmospheric and evolutionary models (which we show employ inappropriate mass-loss rates for late-O main-sequence stars). We show that the most rapid rotator known, HD 191423, is an ON star, and note the implied disparity between O/ON morphology and surface helium abundance; we discuss consequences for the interpretation of spectral morphology in O-type main-sequence stars. We demonstrate a new, purely spectroscopic, method of distance determination for rapid rotators, and thereby confirm that HD 93521 lies at ∼ 2 kpc, and is not, as previously suggested, a low-mass Population II star. Finally, our models contradict earlier claims of strongly differential surface rotation, and are consistent with uniform angular velocity at the surface.

Magnetic Fields in Massive Stars. I. Dynamo Models

Motivated by mounting evidence for the presence of magnetic fields in the atmospheres of normal early-type main-sequence stars, we investigate the various possible modes of dynamo action in their convective core. Working within the framework of mean field electrodynamics, we compute α2 and α2Ω dynamo models and demonstrate that the transition from the former class to the latter occurs smoothly as internal differential rotation is increased. Our models also include a magnetic diffusivity contrast between the core and radiative envelope. The primary challenge facing such models is to somehow bring the magnetic field generated in the deep interior to the stellar surface. We investigate the degree to which thermally driven meridional circulation can act as a suitable transport agent. In all models with strong core-to-envelope magnetic diffusivity contrast—presumably closest to reality— whenever circulation is strong enough to carry a significant magnetic flux, it is also strong enough to prevent dynamo action. Estimates of typical meridional circulation speeds indicate that this regime is likely not attained in the interior of early-type main-sequence stars. Dynamo action then remains highly probable, but an alternate mechanism must be sought to carry the magnetic field to the surface.

Long-term variability of the Be/X-ray binary EXO 2030+375

We present a multi-wavelength study of the Be/X-ray binary system EXO 2030+375. We report that the Be companion is currently in a low-activity phase as indicated by the notable decrease of the infrared and optical emission. If this trend continues the source will lose its circumstellar envelope. Infrared spectroscopy in the IJHK bands is presented for the first time, along with optical and X-ray observations. These infrared spectra agree with the optical companion being an early-type (B0) main-sequence star. When active EXO 2030+375 shows an X-ray outburst at each periastron passage of the neutron star. In addition to the maximum X-ray luminosity displayed at orbital phase ∼0.0, we find a smaller maximum in the light curve at phase ∼0.5. This second intensity peak may be explained if the velocity of the wind is lower than or comparable to the orbital velocity of the neutron star at apastron. We also comment on the relation between the optical/infrared behaviour and the X-ray emission and argue that the X-ray inactive period observed between 1993 August and 1996 April is a result of centrifugal inhibition of accretion of matter rather than a low-activity circumstellar disc.

In-flight degradation results for the UARS SOLSTICE instrument

The SOLar STellar Irradiance Comparison Experiment (SOLSTICE) is a three-channel spectrometer designed for measuring the solar ultraviolet (UV) irradiance from 119 nm to 420 nm with a spectral resolution of 0.1 nm to 0.3 nm. The SOLSTICE is aboard the NASA Upper Atmosphere Research Satellite (UARS), which was launched on 12 September 1991. The degradation of the SOLSTICE sensitivity is primarily tracked in-flight by measuring a set of bright, early-type stars with the same optics and detectors and by changing only slit sizes and integration times. While the Sun changes by 1% in the near-UV and by as much as a factor of 2 in the far-UV, early-type main-sequence stars are not expected to change by more than 1% in the UV for long time periods. The ensemble average of the SOLSTICE stellar observations indicates that these stars are indeed stable to 2% or better. Since the launch of the UARS, the SOLSTICE sensitivity has decreased by a few percent per year. We attribute the degradation primarily to ageing of the photomultiplier tubes for all three channels and to diffusion of layers in the broadband interference filters for the F and N channels. There appears only minor degradation associated with optical contamination, mainly because of the strict use of low-outgassing materials in the SOLSTICE instrument and maintenance of class 10000 clean rooms and oil-free vacuum systems for all pre-flight testing of the instrument.

Infrared Photometry of β Pictoris Type Systems

We obtained small-aperture (4''–5'' diameter) infrared (2–20 μm) photometry of 10 early-type main-sequence stars with infrared excesses from circumstellar dust. These systems possibly exemplify the β Pictoris phenomenon. We observed them with either the NASA Marshall Space Flight Center bolometer array camera (Big Mac) or the Infrared Telescope Facility 2–30 μm single-channel bolometer system. Measurements were obtained in the KLMNQ filters and the narrowband (Δλ ≈ 1 μm) 10 μm filters. We fitted Kurucz photospheric models to the photometric data to determine excess-emission spectra. We report the nondetection of small-aperture circumstellar dust emission from HR 10 and 21 LMi. We confirmed previous nondetections of near-infrared or 10 μm excess emission from 68 Oph, α PsA, and HR 4796A. We did not detect prominent silicate emission from any of the sources. The spectra of γ Oph, σ Her, HR 2174A, β UMa, and ζ Lep show weak 10 μm excesses. We fitted simple models to these data, together with IRAS excess fluxes, to determine plausible distributions of temperature and density of circumstellar dust grains. Significant quantities of these grains around HR 2174A, ζ Lep, and β UMa are at temperatures similar to terrestrial material in the solar system.

Radial Velocities of Six Early Type Evolved Stars

Many early-type main sequence stars are known spectroscopic binaries, while early type spectroscopic binaries with evolved components are rather rare and well-determined masses are available only in some cases. For this reason a program to study stars with suspected variable velocity among early-type evolved stars has been initiated some time ago. In the present paper 54 coude spectra for a sub-group formed of six early-type stars, namely HD 7902, HD 28446, HD 42087, HD 187983, HD 195592 and HD 208501 have been measured and the results of these plates have been presented.

Far-Ultraviolet (912--1900 Angstrom ) Energy Distribution in Early-Type Main-Sequence Stars

view Abstract Citations (13) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Ultraviolet (912--1900 Angstrom ) Energy Distribution in Early-Type Main-Sequence Stars Chavez, M. ; Stalio, R. ; Holberg, J. B. Abstract We present the comparison between observed far-ultraviolet (FUV) fluxes of early-type stars and those predicted by the most recent classical LTE, line-blanketed models of Kurucz. High-quality lUE spectra from the Uniform Low Dispersion Archive (ULDA) of more than 100 stars are selected. The averages of the observed fluxes, normalized to the visual flux, for each spectral class are calculated in three narrow bands centered at 1300, 1500, and 1750 Å and compared with model atmosphere predictions. We found that our results agree better with the models if we use the effective temperature scale of Lesh. lUE fluxes indicate a much lower temperature for B3 stars than reported in several spectral type Teff relationships found in the literature, while for B9 stars our results point to a higher temperature. Using lUE mean fluxes, we derived a new Teff scale for B-type stars. The normalized fluxes obtained using this new scale as input in model atmospheres are compared to the normalized fluxes at 1050 Å of 15 low-reddening early-type stars observed by the Voyager UVS. Voyager fluxes are within a factor of 2 in agreement with the models. Four of these stars match very well the predictions or have slightly lower flux than the models. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176054 Bibcode: 1995ApJ...449..280C Keywords: STARS: ATMOSPHERES; STARS: EARLY-TYPE; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (125) MAST (1) INES (1)

The variability of polarized standard stars

view Abstract Citations (71) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Variability of Polarized Standard Stars Bastien, P. ; Drissen, L. ; Menard, F. ; Moffat, A. F. J. ; Robert, C. ; St-Louis, N. Abstract The linear polarization of 11 out of a total of 13 standard polarized stars has been found to be variable. This implies that these 11 stars, mostly giants and supergiants but also four early-type main-sequence stars, exhibit some level of small, variable, intrinsic polarization. Therefore, they are not suitable as polarized standards for accurate polarimetry. The problem of finding suitable candidates for standard polarized stars is therefore nontrivial and basically still unsolved. The general increase in the level of polarization variability with mean polarization may be partly interstellar in origin, analogous to radioflux scintillations seen in extragalactic radio sources. Publication: The Astronomical Journal Pub Date: March 1988 DOI: 10.1086/114688 Bibcode: 1988AJ.....95..900B Keywords: Early Stars; Extragalactic Radio Sources; Giant Stars; Linear Polarization; Signal To Noise Ratios; Variable Stars; Computational Astrophysics; Error Analysis; Gallium Arsenides; Telescopes; Astrophysics; STARS: MAGNETIC; STARS: PULSATION full text sources ADS | data products SIMBAD (18)

UV time-dependent emission in SY Muscae

Ultraviolet spectra acquired with the International Ultraviolet Explorer (IUE) of SY Mus = HD 10036 on 20 September 1980 and 11 June 1981 indicate a substantial enhancement of UV emission over a nine month period. The general UV flux level appears to have increased by approximately one order of magnitude between the first and second observing epochs. The strong ultraviolet continuum evident throughout the entire IUE spectral range λλ1200-3200 A on 11 June 1981 is closely approximated by a star with Teff = 40,000 K, where previously on 20 September 1980 the continuum distribution presented a more complex structure that is possibly explained by a combination of thermal emission from an early type main sequence star, and nebular recombination emission (Michalitsianos et al. 1981).

Herbig-Haro objects associated with NGC 2264

view Abstract Citations (32) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Herbig-Haro objects associated with NGC 2264. Adams, M. T. ; Strom, K. M. ; Strom, S. E. Abstract Two complexes of emission knots have been discovered near the center of the young cluster NGC 2264. Intensified image dissector scanner spectra of three of these emission knots exhibit line intensities characteristic of Herbig-Haro (HH) objects. In the case of one of these knots, velocities of two condensations were derived from a relatively high-resolution spectrogram. The measured values of +34 km/s and +60 km/s compare with heliocentric velocities of +16.4 km/s for the H II region excited by the early-type main-sequence stars of the cluster, and +22 km/s for the molecular cloud complex located behind the H II region. It appears likely that these knots are HH objects associated with the molecular cloud complex and represent signposts of a star-formation episode triggered by propagation of an ionization front. Publication: The Astrophysical Journal Pub Date: June 1979 DOI: 10.1086/182989 Bibcode: 1979ApJ...230L.183A Keywords: Main Sequence Stars; Nebulae; Star Clusters; Stellar Evolution; Astronomical Spectroscopy; H Ii Regions; High Resolution; Hydrogen Ions; Molecular Excitation; Spectral Energy Distribution; Astrophysics; Herbig-Haro Objects:Open Clusters full text sources ADS | data products SIMBAD (5)

The Evolutionary History of X-Ray Binaries

The most important recent observational discoveries in the field of X-ray binaries are probably those of the slow pulsars and of the winds of normal early-type main-sequence stars. These facts yield key information on the evolutionary history of the X-ray binaries and on the rotational slow-down mechanism for a neutron star in a stellar wind, as will be pointed out in section 3. In the theoretical field, the X-ray binaries have triggered much fundamental work, notably on the detailed processes of mass transfer and on tidal evolution, which will be considered in sections 2, 4 and 5.

Electronographic Photometry of Star Clusters in the Magellanic Clouds-IV THE COLOUR-MAGNITUDE DIAGRAM OF NGC 419*

Electronographic magnitudes and colours of 146 stars in and near the cluster NGC 419 in the Small Magellanic Cloud have been measured to V = 20·9 on electrographs taken with a Spectracon image-converter attached to the f/7·5 focus of the Cerro Tololo 60-in. reflector. Comparison with the photographic photometry of Arp indicates that at distances of r >110″ from the centre of the cluster, Arp's photometric calibration is quite good over the range |$15\cdot5\,\leq\,B,\,V\,\leq\,20\cdot1$| . However, for r ≤110″ the photographic measures are affected by background light from the faint, unresolved members of the cluster. The upper portion of the colour–magnitude diagram of NGC 419 consists of a red-giant branch which extends from V = 15·9 and B–V = + 2·0 to V = 19·8 and B-V = +o·7 and resembles that in Kron 3. However, fainter than V ≃ 19·8 the diagram is confused by the presence in the field of many early-type main-sequence stars belonging to the main body of the Small Magellanic Cloud. Consequently, it is not clear whether NGC 419 is: (1) a cluster like Kron 3 but in which there is a gap in the giant-branch between about V = 19·8 and V = 20·3, or (2) is a cluster more like NGC 2209 with a red-giant branch, a small Hertzsprung gap, and an evolved mainsequence whose upper end appears at about V = 20·2 and B–V = +0·25. One extremely red star was measured which is probably a carbon star belonging to the general population of the Small Magellanic Cloud.

Neutral Interstellar Hydrogen in the Solar Neighbourhood

WE have listed1 three requirements that have to be fulfilled for determinations of the mean volume density of the interstellar neutral hydrogen, nH, from Lyman α absorption measurements, (a) Stars that are likely to be surrounded by circumstellar envelopes, such as early-type giants and supergiants, have to be excluded, because circumstellar envelopes can give rise to a Lyman α emission sufficient to affect the absorption measurements by a significant amount. The best candidates for a meaningful measurement are therefore early-type main-sequence stars or horizontal branch stars. (b) Good spectral resolution (Δλ ≤ 2 A) is needed to resolve Lyman α from other strong nearby absorption features. (c) Observations of a large number of stars are needed in order to reduce the error in the determination of nH due to the rather large uncertainties in the measured equivalent widths and in the adopted distance to each star.

Absorption Line Strengths in Rotating Stars

Computations of equivalent widths of the absorption lines of He I 4471 and Mg II 4481 in early type main sequence stars with various rotational velocities and viewed at different angles of inclination are presented. The results of this investigation suggest that it is impracticable to use absorption line strengths to determine angles of inclination.