Early B-type Main-sequence Stars Research Articles

Searching for OB-type pre-supernova binary companions inside supernova remnants

Abstract We searched for OB-runaway stars inside supernova remnants (SNRs) as a pre-supernova binary companion. As the majority of massive stars are found in close binary systems, a runaway star ejected by the orbital energy after the supernova (SN) is expected to be found. Considering a binary mass fraction, q=0.25 − 1.0, the runaway star is likely to have an OB spectral type. We selected 12 SNRs at Galactic longitudes 109○ − 189○. Using Gaia astrometry we selected stars having consistent distances with those of the SNRs and showing a peculiar proper motion directed away from the central region of the SNRs. We also determined the radial distribution of the extinction towards the SNRs and estimated the spectral types of the OB-runaway candidates through Gaia and 2MASS photometry. We found two candidates among 12 SNRs. By spectroscopic observations, Gaia DR3 195632152560621440 inside SNR G166.0+4.3 was found to be an evolved A3 type star which cannot be the pre-SN binary companion to the progenitor. Gaia DR3 513927750767375872 inside SNR HB 3 is the only OB-runaway candidate (2-D space velocity of 33.3±3.3 km s−1). Based on photometric study, the star can be an early B-type main sequence star with a slightly higher extinction relative to the SNR, however, it might instead be an evolved A-type star at the same distance. The other 11 SNRs do not host an OB-runaway star within their central regions. Although the runaway search was performed in a large interval of distance and extinction, we also estimated distances to the SNRs.

Ultraviolet Detection of the Binary Companion to the Type IIb SN 2001ig

Abstract We present HST/WFC3 ultraviolet imaging in the F275W and F336W bands of the Type IIb SN 2001ig at an age of more than 14 years. A clear point source is detected at the site of the explosion, with m F275W = 25.39 ± 0.10 and m F336W = 25.88 ± 0.13 mag. Despite weak constraints on both the distance to the host galaxy NGC 7424 and the line-of-sight reddening to the supernova, this source matches the characteristics of an early B-type main-sequence star with 19,000 < T eff < 22,000 K and . A BPASS v2.1 binary evolution model, with primary and secondary masses of 13 M ⊙ and 9 M ⊙, respectively, is found to simultaneously resemble, in the Hertzsprung–Russell diagram, both the observed location of this surviving companion, and the primary star evolutionary endpoints for other Type IIb supernovae. This same model exhibits highly variable late-stage mass loss, as expected from the behavior of the radio light curves. A Gemini/GMOS optical spectrum at an age of 6 years reveals a narrow He ii λ4686 emission line, indicative of continuing interaction with a dense circumstellar medium at large radii from the progenitor. We review our findings on SN 2001ig in the context of binary evolution channels for stripped-envelope supernovae. Owing to the uncrowded nature of its environment in the ultraviolet, this study of SN 2001ig represents one of the cleanest detections to date of a surviving binary companion to a Type IIb supernova.

The N/O Ratio in Early B-Type Main Sequence Stars as an Indicator of Their Evolution

It is shown that, in the case of early B-type main-sequence stars, of the three ratios N/C, C/O, and N/O which are regarded as indicators of stellar evolution, the ratio N/O is more reliable since it seems to be insensitive to overionization of the NII and OII ions. On the other hand, the N/C and C/O ratios, which include carbon, may contain systematic errors for stars with Teff > 18500 K because of neglected overionization of CII ions. The ratio N/O is studied in the atmospheres of 46 early main-sequence B stars. These values of N/O are examined as functions of the effective temperature, age, rotation speed, and mass of the stars. Most early B-stars in the main sequence are found to have [N / O] = 0 , which indicates that N/O varies little during the main sequence stage, and this result is independent of the basic parameters listed above. There are two explanations for the large number of stars with [N / O] = 0 : it is predicted theoretically that for an initial rotation velocity V0 < 100 km/s, N/O varies little toward the end of the main sequence stage ([N/O] < 0.2) and observations show that most early main-sequence B-stars do actually have low initial rotation velocities V0. The few early main-sequence B-stars with higher [N/O] = 0.4-0.8 correspond to models with rotation velocities V0 = 200-300 km/s. This conclusion is consistent with earlier data for stars with the same masses in a later stage of evolution: the AFG-supergiant and bright giant stage.

PROJECTED ROTATIONAL VELOCITIES AND STELLAR CHARACTERIZATION OF 350 B STARS IN THE NEARBY GALACTIC DISK

Projected rotational velocities (vsini) are presented for a sample of 350 early B-type main sequence stars in the nearby Galactic disk. The stars are located within ~1.5 kpc from the Sun, and the great majority within 700 pc. The analysis is based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m telescope at the Las Campanas Observatory in Chile.Spectral types were estimated based on relative intensities of some key line absorption ratios and comparisons to synthetic spectra. Effective temperatures were estimated from the reddening-free Q index, and projected rotational velocities were then determined via interpolation on a published grid that correlates the synthetic full width at half maximum of the He I lines at 4026, 4388 and 4471 A with vsini. As the sample has been selected solely on the basis of spectral types it contains an selection of B stars in the field, in clusters, and in OB associations. The vsini distribution obtained for the entire sample is found to be essentially flat for vsini values between 0-150 km/s, with only a modest peak at low projected rotational velocities. Considering subsamples of stars, there appears to be a gradation in the vsini distribution with the field stars presenting a larger fraction of the slow rotators and the cluster stars distribution showing an excess of stars with vsini between 70 and 130 km/s. Furthermore, for a subsample of potential runaway stars we find that the vsini distribution resembles the distribution seen in denser environments, which could suggest that these runaway stars have been subject to dynamical ejection mechanisms.

Nitrogen chronology of massive main sequence stars

Rotational mixing in massive main sequence stars is predicted to monotonically increase their surface nitrogen abundance with time. We use this effect to design a method for constraining the age and the inclination angle of massive main sequence stars, given their observed luminosity, effective temperature, projected rotational velocity and surface nitrogen abundance. This method relies on stellar evolution models for different metallicities, masses and rotation rates. We use the population synthesis code STARMAKER to show the range of applicability of our method. We apply this method to 79 early B-type main sequence stars near the LMC clusters NGC 2004 and N 11 and the SMC clusters NGC 330 and NGC 346. From all stars within the sample, 17 were found to be suitable for an age analysis. For ten of them, which are rapidly rotating stars without a strong nitrogen enhancement, it has been previously concluded that they did not evolve as rotationally mixed single stars. This is confirmed by our analysis, which flags the age of these objects as highly discrepant with their isochrone ages. For the other seven stars, their nitrogen and isochrone ages are found to agree within error bars, what validates our method. Constraints on the inclination angle have been derived for the other 62 stars,with the implication that the nitrogen abundances of the SMC stars, for which mostly only upper limits are known, fall on average significantly below those limits. Nitrogen chronology is found to be a new useful tool for testing stellar evolution and to constrain fundamental properties of massive main sequence stars. A web version of this tool is provided.

The Long‐Period, Massive Binaries HD 37366 and HD 54662: Potential Targets for Long‐Baseline Optical Interferometry

We present the results from an optical spectroscopic analysis of the massive stars HD 37366 and HD 54662. We find that HD 37366 is a double-lined spectroscopic binary with a period of 31.8187 +/- 0.0004 days, and HD 54662 is also a double lined binary with a much longer period of 557.8 +/- 0.3 days. The primary of HD 37366 is classified as O9.5 V, and it contributes approximately two-thirds of the optical flux. The less luminous secondary is a broad-lined, early B-type main-sequence star. Tomographic reconstruction of the individual spectra of HD 37366 reveals absorption lines present in each component, enabling us to constrain the nature of the secondary and physical characteristics of both stars. Tomographic reconstruction was not possible for HD 54662; however, we do present mean spectra from our observations that show that the secondary component is approximately half as bright as the primary. The observed spectral energy distributions (SEDs) were fit with model SEDs and galactic reddening curves to determine the angular sizes of the stars. By assuming radii appropriate for their classifications, we determine distance ranges of 1.4 - 1.9 and 1.2 - 1.5 kpc for HD 37366 and HD 54662, respectively.

Rotation of Early B-type Stars in the Large Magellanic Cloud: The Role of Evolution and Metallicity

AbstractI present measurements of the projected rotational velocities of a sample of 100 early B-type main-sequence stars in the Large Magellanic Cloud (LMC). This is the first extragalactic study of the distribution of stellar rotational velocities. The sample is drawn from two sources: from the vicinity of the main-sequence turnoff of young clusters (ages 1–3 × 107 yr) and from the general field. I find that the cluster population exhibits significantly more rapid rotation than that seen in the field. I have drawn analogous Galactic cluster and field samples from the literature. Comparison of these samples reveals the same effect. I propose that the observed difference between cluster and field populations can be explained by a scenario of evolutionary enhancement of the surface angular momentum over the main-sequence lifetime. A comparison is made between the cluster and field populations of the LMC and the Galaxy in order to explore the effects of metallicity. This shows that the stars of the LMC are more rapid rotators than their Galactic counterparts.

A Spectroscopic Search for Non-Radial Pulsations in Early B-Type Stars

It is known that early-B type main-sequence stars can be unstable against pulsations induced by the κ mechanism. Traditionally, radial and low-degree non-radial β Cephei-like pulsations have been detected photometrically; nowadays radial, low-degree and intermediate-degree pulsations can be detected spectroscopically through line-profile variations (LPVs). Whereas most of the brightest early-B type stars have been photometrically monitored for variability (72 of the 541 O9.5-B3 III-V stars in the BSC are classified as β Cephei stars), a systematic LPV survey to look for pulsational behaviour was lacking. Here we present some preliminary results of the southern part of our Spectroscopic survey of early-B type stars. We have made a high spectral resolution survey of the Si III λ 4552, 4567, 4574 Å triplet in early-B type stars in order to look for LPVs due to non-radial pulsation (NRP). Our sample consists of 82 southern O9.5-B3 stars with spectral classes III-V, taken from the BSC; 27% of all O9.5-B2.5 III-V stars in the BSC were sampled. This sample is not fully representative for all nearby early-B stars, because we concentrated on stars with high rotational velocities, because many of the stars are members of the Sco-Cen associations, and because we focused on the β Cephei stars in the BSC. As much as 40% of the β Cephei stars in the BSC with v sin г ≥ 25 km s-1 were sampled. The observations were carried out with the ESO CAT telescope at resolution R∼65000. Our targets were observed during 6 runs in September 1995 – June 1998. Some stars were observed many times, but most targets were observed only once or twice.

The chemical composition towards the Galactic anti-centre. I. Observations and model atmosphere analyses

High resolution spectra of six early B-type main-sequence stars having galactocentric distances of between 10 and 18 kpc are presented. We list the equivalent widths for the metal lines and illustrate their hydrogen and helium line profiles. The stars are analysed using LTE line-blanketed model atmosphere techniques to derive atmospheric parameters and surface chemical compositions. All six stars have similar effective temperatures and surface gravities, allowing a reliable comparison of their metal abundances and distances. Significant variations in the photospheric abundances are evident and we discuss the need for a more detailed line-by-line differential analysis to exactly quantify the differences. This will be presented in a companion paper (Smartt et al. 1996).

Coronal gas in the galactic halo: ORFEUS obser vations of NGC 346 #1

We present high resolution ( λ δλ = 3000 ) observations of the Small Magellanic Cloud (SMC) star NGC 346 #1 near the O vi resonance doublet (1031.9, 1037.6 Å). The data were collected with the Berkeley extreme/far ultraviolet (EUV/FUV) spectrometer in the ORFEUS telescope aboard the space platform Astro-SPAS during the mission of space shuttle Discovery flown in September 1993. The O vi feature shows broad absorption centered near ν LSR ≈ +140 km/sec. This is almost certainly associated with the OB cluster NGC 346 itself, which also shows deep absorption at Si iv and C iv at comparable velocities. An absorption feature corresponding to O vi near ν LSR ≈ −230 km/sec is present in our data, but this feature is caused by contamination from a second star, probably an early B-type main sequence star at lower local standard of rest (LSR) velocity, which is also present in the 20″ circular aperture. Between these velocity extrema lies one broad feature ( b ≈ 70 km/sec) or multiple narrower features, with a combined column density of about 1.4 × 10 14 cm −2. Assuming an exponential distribution of O vi with the in-plane density of Jenkins /1/, we infer a scale height, H 0, of no more than ≈1.2 kpc toward NGC 346 #1. Our O vi column density is about a factor of 4 below the predictions of cooling fountain models, but our low scale height is consistent with more recent predictions /2/.