arXiv subjects

L. Drissen

Publications and source records attributed to L. Drissen.

24 records · Page 2Linked to original sources

arXiv2008

Discovery of two magnetic massive stars in the Orion Nebula Cluster: a clue to the origin of neutron star magnetic fields?

The origin of the magnetic fields in neutron stars, and the physical differences between magnetars and strongly magnetised radio pulsars are still under vigorous debate. It has been suggested that the properties of the progenitors of neutron stars (the massive OB stars), such as rotation, magnetic fields and mass, may play an important role in the outcome of core collapse leading to type II SNe. Therefore, knowing the magnetic properties of the progenitor OB stars would be an important asset for constraining models of stellar evolution leading to the birth of a neutron star. We present here the beginning of a broad study with the goal of characterising the magnetic properties of main sequence massive OB stars. We report the detection of two new massive magnetic stars in the Orion Nebula Cluster: Par 1772 (HD 36982) and NU Ori (HD 37061), for which the estimated dipole polar strengths, with 1 sigma error bars, are 1150 (+320,-200) G and 650 (+220,-170) G respectively.

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arXiv2007

Exploring the origin of neutron star magnetic field: magnetic properties of the progenitor OB stars

Ferrario & Wickramasinghe (2006) explored the hypothesis that the magnetic fields of neutron stars are of fossil origin. In this context, they predicted the field distribution of the progenitor OB stars, finding that 5 per cent of main sequence massive stars should have fields in excess of 1kG. We have carried out sensitive ESPaDOnS spectropolarimetric observations to search for direct evidence of such fields in all massive B- and O-type stars in the Orion Nebula Cluster star-forming region. We have detected unambiguous Stokes V Zeeman signatures in spectra of three out of the eight stars observed (38%). Using a new state-of-the-art Bayesian analysis, we infer the presence of strong (kG), organised magnetic fields in their photospheres. For the remaining five stars, we constrain any dipolar fields in the photosphere to be weaker than about 200G. Statistically, the chance of finding three ~kG fields in a sample of eight OB stars is quite low (less than 1%) if the predictions of Ferrario & Wickramasinghe are correct. This implies that either the magnetic fields of neutron stars are not of fossil origin, that the flux-evolution model of Ferrario & Wickramasinghe is incomplete, or that the ONC has unusual magnetic properties. We are undertaking a study of other young star clusters, in order to better explore these possibilities.

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arXiv2003

Gemini observations of Wolf-Rayet Stars in the Local Group Starburst Galaxy IC10

We present Gemini-N GMOS and CFHT MOS spectroscopy of Wolf-Rayet candidates in the Local Group dwarf galaxy IC10 that were previously identified by Massey et al. and Royer et al. From the present spectroscopic survey, the WC/WN ratio for IC10 remains unusually high, given its low metallicity, although none of the WC9 stars suspected from narrow-band imaging are confirmed. Our spectroscopy confirms 9 newly discovered Wolf-Rayet candidates from Royer et al., whilst spectral types of 14 Wolf-Rayet stars previously observed by Massey & Armandroff are refined here. In total, there are 26 spectroscopically confirmed Wolf-Rayet stars in IC~10. All but one of the fourteen WC stars are WC4-6 stars, the exception being # 10 from Massey et al., a broad-lined, apparently single WC7 star. There are a total of eleven WN stars, which are predominantly early WN3-4 stars, but include a rare WN10 star, #8 from Royer et al. #5 from Massey et al. is newly identified as a transition WN/C star. Consequently, the WC/WN ratio for IC10 is 14/11=1.3, unusually high for a metal-poor galaxy. Re-evaluating recent photometric data of Massey & Holmes, we suggest that the true WC/WN ratio may not be as low as 0.3. Finally, we present ground-based finding charts for all confirmed WR stars, plus HST/WFPC2 charts for twelve cases.

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arXiv2000

Physical parameters of erupting Luminous Blue Variables: NGC 2363-V1 caught in the act

A quantitative study of the Luminous Blue Variable NGC 2363-V1 in the Magellanic galaxy NGC 2366 (D = 3.44 Mpc) is presented, based on ultraviolet and optical HST/STIS spectroscopy. Contemporary WFPC2 and WHT imaging reveals a modest V-band brightness increase of ~ 0.2 mag per year between 1996 January and 1997 November, reaching V=17.4 mag, corresponding to Mv=-10.4 mag. Subsequently, V1 underwent a similar decrease in V-band brightness, together with a UV brightening of 0.35 mag from 1997 November to 1999 November. The optical spectrum of V1 is dominated by H emission lines, with Fe II, He I and Na I also detected. In the ultraviolet, a forest of Fe absorption features and numerous absorption lines typical of mid-B supergiants are observed. From a spectral analysis with the non-LTE, line-blanketed code of Hillier & Miller (1998), we derive stellar parameters of T*=11kK, R*=420Ro, log(L/Lo)=6.35 during 1997 November, and T*=13kK, R*=315Ro, log(L/Lo)=6.4 for 1999 July. The wind properties of V1 are also exceptional, with Mdot ~ 4.4 x 10e-4 Mo/yr and $v_{\infty} ~ $300 km/s, allowing for a clumped wind (filling factor = 0.3), and assuming H/He ~ 4 by number. The presence of Fe lines in the UV and optical spectrum of V1 permits an estimate of the heavy elemental abundance of NGC 2363 from our spectral synthesis. Although some deficiencies remain, allowance for charge exchange reactions in our calculations supports a SMC-like metallicity, that has previously been determined for NGC 2363 from nebular oxygen diagnostics. Considering a variety of possible progenitor stars, V1 has definitely undergone a giant eruption, with a substantial increase in stellar luminosity, radius, and almost certainly mass-loss rate.

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arXiv1999

The Star Formation History of the Starburst Region NGC 2363 and its Surroundings

We present HST optical images and UV spectra, as well as ground-based near-infrared images of the high surface brightness giant HII region NGC 2363 (NGC 2366-I) and its surroundings. The massive star content of the southern end of the dwarf irregular galaxy NGC 2366 is investigated, with an emphasis on Wolf-Rayet and red supergiant stars, and we attempt the reconstruction of the time sequence of the most recent episode of massive star formation at the southwestern tip of the galaxy. We show that the most massive super cluster A of NGC 2363 is still embedded in dust; from the photoevaporative erosion or ``cleaning'' time scale of the associated cloud, we infer its age to be 1 Myr or less. We conclude that the star-forming complex NGC 2366-I and II is a good example of a multiple stage starburst with a characteristic age decreasing from 10 Myr to less than 1 Myr over a linear scale of 400 pc. The age sequence of the stars and the gas kinematics suggest that these powerful star formation episodes are being triggered by a small passing-by satellite.

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arXiv1997

Stellar Populations and Variable Stars in the Core of the Globular Cluster M5

We report the discovery of a variable blue straggler in the core of the globular cluster M5, based on a 12-hour long series of images obtained with the Planetary Camera aboard the Hubble Space Telescope. In addition, we present the light curves of 28 previously unknown or poorly studied large-amplitude variable stars (all but one are RR Lyrae) in the cluster core. A (V, U-I) color-magnitude diagram shows 24 blue stragglers within 2 core radii of the cluster center. The blue straggler population is significantly more centrally concentrated than the horizontal branch and red giant stars.

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