Internationalisation of R&amp;D: A Brief Survey of the Literature

Dachs, Bernhard

doi:10.4337/9781783470907.00008

Cited by 124 publications

(192 citation statements)

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“…Similar segregations of Be and normal B stars were also found by Keller et al (1999). This could be due to one or more of stellar evolution, light scattering in the circumstellar disk (Dachs et al 1988), and gravitational darkening linked to the rapid rotation of Be stars . Of these, evolutionary differences are the least likely since the comparisons are made for stars in the same open clusters and of similar spectral type.…”

Section: Colour Excesses Of Emission-line Starssupporting

confidence: 56%

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

Martayan

Baade

Fabregat³

2010

A&A

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Context.A fair fraction of all single early-type stars display emission lines well before their supergiant phase. Very rapid rotation is necessary for these stars to form rotationally supported decretion disks. However, it is unknown whether and which other parameters may be important. Aims. We assess the roles of metallicity and evolutionary age in the appearance of the so-called Be phenomenon. Methods. Slitless CCD spectra were obtained covering the bulk (about 3 square degrees) of the Small Magellanic Cloud. For Hα line-emission twice as strong as the ambient continuum, the survey is complete to spectral type B2/B3 on the main sequence. About 8120 spectra of 4437 stars were searched for emission lines in 84 open clusters, and 370 emission-line stars were found, among them at least 231 close to the main sequence. For 176 of them, photometry is available from the OGLE database. For comparison with a higher-metallicity environment, the Galactic sample of the photometric Hα survey by McSwain & Gies (2005, ApJS, 161, 118) was used.Results. Among early spectral sub-types, Be stars are more frequent by a factor ∼3-5 in the SMC than in the Galaxy. The distribution with spectral type is similar in both galaxies, i.e., not strongly dependent on metallicity. The fraction of Be stars does not seem to vary with local star density. The Be phenomenon mainly sets in towards the end of the main-sequence evolution (this trend may be more pronounced in the SMC); but some Be stars already form with Be-star characteristics. In small subsamples (such as single clusters), even if they appear identical, the fraction of emission-line stars can deviate drastically from the mean. Conclusions. In all probability, the fractional critical angular rotation rate, Ω/Ω c , is one of the main parameters governing the occurrence of the Be phenomenon. If the Be character is only acquired during the course of evolution, the key circumstance is the evolution of Ω/Ω c , which is not only dependent on metallicity but differently so for different mass ranges. As a result, even if the Be phenomenon is driven basically by a single parameter (namely Ω/Ω c ), it can assume a complex multi-parametric appearance. The large cluster-to-cluster differences, which seem stronger than all other variations, serve as a caveat that this big picture may undergo significant second-order modulations (e.g., pulsations, initial angular momentum, etc.).

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Section: Colour Excesses Of Emission-line Starssupporting

confidence: 56%

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

Martayan

Baade

Fabregat³

2010

A&A

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“…To test for other Be star characteristics in δ Sco we checked for correlations between H alpha line parameters like v sin i vs. fullwidth, v sin i vs. halfwidth, peak to continuum intensity ratio vs. equivalent width etc. (Andrillat & Fehrenbach 1982;Banerjee et al 2000;Dachs et al 1986) and found it to exhibit standard Be star behaviour.…”

Section: Resultsmentioning

confidence: 99%

Near infra-red and optical spectroscopy of Delta Scorpii

Banerjee

Janardhan

Ashok

2001

A&A

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Abstract. Spectroscopic observations of δ Sco in the near IR and optical wavelengths, obtained shortly after its recent periastron passage are reported. The IR spectra are dominated by the HI Paschen and Brackett lines. An IR excess is seen which can be attributed to free-free emission from a circumstellar shell. The observed Paschen and Brackett line intensities deviate significantly from Case B predictions and indicate optical depth effects. These effects are modeled. The overall spectral behaviour of δ Sco at present is that of a typical Be star.

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“…It is generally observed in classical Be stars and originates in the ionized part of a flattened CS envelope (CSE, Waters et al 1987;Dachs et al 1988). The Be phenomenon is known to manifest itself especially towards early-type Be stars.…”

Section: Variable Continuum Bf-ff Radiationmentioning

confidence: 99%

“…As the ionized CSE produces both the emission line spectrum and the optical color excess of the classical Be stars, these two quantities are related. An approximate relation between E(B − V) CS and the EW of the Hα line for Galactic Be stars was derived by Dachs et al (1988), which is shown in Fig. 4 together with the Hα emission and CS color excess of the ELHCs.…”

Section: Variable Continuum Bf-ff Radiationmentioning

confidence: 99%

On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud

Wit¹,

Beaulieu

Lamers

et al. 2005

A&A

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Abstract. We investigate a sample of 18 Large Magellanic Cloud Herbig Ae/Be candidate stars looking at their (1) spectral types (2) brightness variability mechanism and (3) near infra-red JHK emission. We find that the majority of the target stars have Hα emission, are of spectral type early-to mid-B and lack strong JHK excess emission. Their Balmer decrements are found to be similar to that of Galactic Be stars in general. Their erratic brightness variability is evaluated by using the observed optical color excess and the color gradient from the light curves and is subsequently interpreted as being due to variable dust obscuration or variable bf-ff emission from circumstellar ionized gas. For approximately half of the target stars in our sample the type of variability seems to be dissimilar to the mechanism involving bound-free and free-free emission, but could be interpreted as caused by variable dust obscuration, as we have proposed in earlier studies. It is therefore suggested that they are pre-main sequence objects, despite the fact that they nearly all lack thermal dust emission in the near infra-red; mid/far infra-red observations for these objects are warranted. One star is observed to have JHK excess emission and an inspection of its 7.5 year MACHO light curve confirms its erratic photometric behavior. The object displays deep photometric minima with a quasi-period of 191.3 days, as generally seen in the Galactic pre-main sequence subgroup of the UX Orionis stars.

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Internationalisation of R&D: A Brief Survey of the Literature

Cited by 124 publications

References 0 publications

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

Near infra-red and optical spectroscopy of Delta Scorpii

On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud

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