scholarly journals H$_{\bf\alpha}$ emission in pre-main sequence stars. I. An atlas of line profiles

1996 ◽  
Vol 120 (2) ◽  
pp. 229-256 ◽  
Author(s):  
Bo Reipurth ◽  
A. Pedrosa ◽  
M. T.V.T. Lago
Keyword(s):  
Main Sequence ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Alpha Emission

scholarly journals NLTE spectroscopic analysis of the 3He anomaly in subluminous B-type stars

2018 ◽  
Vol 618 ◽  
pp. A86 ◽  
Author(s):  
D. Schneider ◽  
A. Irrgang ◽  
U. Heber ◽  
M. F. Nieva ◽  
N. Przybilla
Keyword(s):  
Main Sequence ◽  
Spectroscopic Analysis ◽  
Stellar Atmosphere ◽  
Atomic Diffusion ◽  
Horizontal Branch ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Sdb Stars ◽  
Non Local ◽  
First Time

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the 3He stars, which exhibit a remarkable enrichment of 3He with respect to 4He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Recent surveys uncovered 11 3He sdBs. The 3He anomaly is not due to thermonuclear processes, but caused by atomic diffusion in the stellar atmosphere. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known 3He BHBs and nine known 3He sdBs to determine their isotopic helium abundances and 4He/3He abundance ratios. We redetermined their atmospheric parameters and analyzed selected He I lines, including λ4922 Å and λ6678 Å, which are very sensitive to 4He/3He. Most of the 3He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48° 2721 is reclassified as a BHB star because of its low temperature (Teff = 20700 K). Whereas 4He is almost absent (4He/3He < 0.25) in most of the known 3He stars, other sample stars show abundance ratios up to 4He/3He ∼2.51. A search for 3He stars among 26 candidate sdBs from the ESO SPY survey led to the discovery of two new 3He sdB stars (HE 0929–0424 and HE 1047–0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48° 2721).

scholarly journals Non-Local Convection Models for Stellar Atmospheres and Envelopes

2003 ◽  
Vol 210 ◽  
pp. 143-156
Author(s):  
F. Kupka
Keyword(s):  
Numerical Simulations ◽  
Main Sequence ◽  
Stellar Atmospheres ◽  
Surface Velocity ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Solar Type ◽  
Non Local ◽  
Stress Models ◽  
Do So

We present an overview of the concepts underlying advanced non-local Reynolds stress models of turbulent convection and review a comparison of this approach with a series of numerical simulations of fully compressible convection. We then discuss results from applications of the model to complete envelopes of A-type main sequence stars. The non-local model reproduces surface velocities in agreement with the lower limit of observed macro- and microturbulence velocities of A-star photospheres, the asymmetry of the surface velocity field as inferred from spectral line profiles, and the overall structure of the photospheric and subphotospheric convection zones, as predicted by the most recent numerical simulations available for these stars. Traditionally, local models of convection are unable to do so. We conclude with a brief survey of extensions of the model which are interesting for other applications such as atmospheres of solar type stars and overshooting below deep convective envelopes or above the core in massive stars.

scholarly journals Magnetospheric accretion in the intermediate-mass T Tauri star HQ Tauri

2020 ◽  
Vol 642 ◽  
pp. A99 ◽  
Author(s):  
K. Pouilly ◽  
J. Bouvier ◽  
E. Alecian ◽  
S. H. P. Alencar ◽  
A.-M. Cody ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
Main Sequence ◽  
T Tauri Stars ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
Accretion Process ◽  
T Tauri ◽  
Magnetospheric Accretion

Context. Classical T Tauri stars are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M⊙). Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT. Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, rcor ∼ 3.5 R⋆. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined. Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric accretion process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tauri.

scholarly journals Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars

1988 ◽  
Vol 132 ◽  
pp. 309-312
Author(s):  
David A. Bohlender ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Hot Stars ◽  
Surface Magnetic Field ◽  
Peculiar Stars ◽  
Ap Stars

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

scholarly journals A Survey of Northern Bok Globules and the Mon OB1/RI Association For H-Alpha Emission Stars

1984 ◽  
Vol 78 ◽  
pp. 283-286
Author(s):  
Katsuo Ogura ◽  
Tatsuhiko Hasegawa
Keyword(s):  
Star Formation ◽  
Main Sequence ◽  
Direct Approach ◽  
Main Sequence Stars ◽  
Radio Observations ◽  
Alpha Emission ◽  
Gum Nebula ◽  
Bok Globules ◽  
Emission Stars ◽  
Relationship Of

The most direct approach to the suggested relationship of Bok globules to star formation is to search for actual spots of star birth in or around them. But no protostellar objects have hitherto been found by infrared and/or radio observations. Optically relevant to this are the discoveries of some Herbig-Haro objects and pre-main sequence stars associated with cometary globules inside the Gum Nebula (Reipurth 1983).

scholarly journals Sodium Ionization in T-Tauri Stars

1989 ◽  
Vol 120 ◽  
pp. 304-304
Author(s):  
A. Natta ◽  
C. Giovanardi
Keyword(s):  
Mass Loss ◽  
Main Sequence ◽  
T Tauri Stars ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
T Tauri

We present the results of a study of the sodium ionization and excitation in the winds of low-luminosity, pre-main-sequence stars. Line profiles for the Nal doublet at 5990,5986 Åare discussed and compared with the observations for those T Tauri stars with P-Cygni profiles. We find that the observed shape of the lines put significative constraints on the rate of mass-loss (M ≥ 3 x 10-8M⊙/yr).

scholarly journals Optical and H-Alpha Emission from Unresolved Red Dwarf Stars in the Galaxy

1990 ◽  
Vol 139 ◽  
pp. 107-109
Author(s):  
G. Szécsényi-Nagy
Keyword(s):  
Main Sequence ◽  
Stellar Content ◽  
Main Sequence Stars ◽  
M Dwarfs ◽  
Dwarf Stars ◽  
Alpha Emission ◽  
Cool Stars ◽  
Red Dwarfs ◽  
The Galaxy ◽  
Red Dwarf Stars

Although many of the nearest dim and cool stars (the red dwarfs) were catalogued in the early sixties, the majority of the astronomers did not realize that these objects provide almost nine tenths of all the stars in our Galaxy. In fact 90-95% of the stars in the solar vicinity (r ≤ 25 pc) are main sequence stars, and at least 80% of them are M dwarfs. Outside the main sequence (MS) one can find a few subdwarfs and somewhat more white dwarfs, but the contribution of this latter type is not known precisely. Estimates range between 4% and 8%. The relative frequency of giants and supergiants can not be determined from the census of the local star population because they are not represented in a statistically meaningful number. But investigations of much greater cosmic volumes demonstrate that luminosity classes Ia, Ib, II, III, and IV altogether contribute fewer than 1% of the stellar content of the Galaxy.

scholarly journals Rotational Studies of Lower Main Sequence Stars

1980 ◽  
Vol 5 ◽  
pp. 827-830
Author(s):  
Myron A. Smith
Keyword(s):  
Fourier Transform ◽  
Main Sequence ◽  
Velocity Fields ◽  
Slow Rotation ◽  
Line Profiles ◽  
Late Type ◽  
Main Sequence Stars ◽  
Rotational Modulation ◽  
Spotted Star ◽  
The Fourier Transform

This review summarizes the techniques and limitations involved in determining small rotational velocities in late-type stars. Recent results from photoelectric line profiles of field main sequence G stars will also be presented.Historically three techniques have been used to measure stellar rotational velocities: (1.) calibration of a “clean” line width in terms of computed profile models, (2.) use of the Fourier transform frequency domain (dissection of rotation and turbulent velocity fields from the thermal profile), and (3.) monitoring of the rotational modulation of a chromospherically active or spotted star. The results discussed below concern technique (2.). Technique (3.) has not been exploited much yet but holds considerable promise for the future, in particular for the measurement of ultra-slow rotation.

scholarly journals C IV resonance line profiles in O stars

1979 ◽  
Vol 83 ◽  
pp. 95-98
Author(s):  
S. B. Parsons ◽  
J. D. Wray ◽  
K. G. Henize ◽  
G. F. Benedict
Keyword(s):  
Resonance Line ◽  
Main Sequence ◽  
Uv Spectra ◽  
Line Profiles ◽  
Early Type ◽  
Main Sequence Stars ◽  
Objective Prism ◽  
Early Type Stars

The S-019 experiment on Skylab (cf. Henize et al. 1975; “Paper I”) recorded far UV spectra in about 160 4° x 5° fields, covering 10% of the sky, on 101 film with a 15 cm aperture objective-prism telescope. Several hundred early-type stars were observed in the vicinity of 1550 Å with a resolution between 3 and 4 Å, as well as thousands of stars at longer wavelengths and correspondingly lower resolution. An atlas of spectra for types 04 to B5 is illustrated in Paper I. That figure shows that the P Cygni profile is a characteristic of all supergiants earlier than B3 and main sequence stars earlier than 08.

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