arXiv subjects

Michael S. Bessell

Publications and source records attributed to Michael S. Bessell.

46 records · Page 3Linked to original sources

arXiv2010

Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis

We present multi-epoch medium-resolution observations of two M4.5 candidate members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six months of observations both stars exhibited variations in their H-alpha line profiles on timescales of days to months, with at least one episode of substantial activity attributable to accretion from a circumstellar disk. We derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of ~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters are likely incomplete and that gas dissipation timescales calculated from the fraction of accreting objects are underestimates.

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arXiv2010

First detection of a low-mass stellar halo around the young open cluster Eta Chamaeleontis

We have identified several lithium-rich low-mass (0.08<M<0.3 Msun) stars within 5.5 deg of the young open cluster Eta Chamaeleontis, nearly four times the radius of previous search efforts. Of these stars we propose 4 new probable cluster members, and 3 possible members requiring further investigation. These findings are consistent with a dynamical origin for the current configuration of the cluster, without the need to invoke an abnormal Initial Mass Function deficient in low-mass objects. Candidates were selected on the basis of DENIS and 2MASS photometry, NOMAD astrometry and extensive follow-up spectroscopy.

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arXiv2008

Towards a complete census of young stars in the solar neighbourhood with SkyMapper

In this contribution we outline plans for identifying and characterising numerous young, low-mass stars within 150 pc of the Sun using the new SkyMapper telescope and Southern Sky Survey. We aim to learn more about the star formation history of the solar neighbourhood over the past 5-50 Myr, the dispersal processes involved, as well as testing pre-main sequence evolutionary models and the universality of the stellar Inital Mass Function. Searching for the dispersed halo of low-mass objects predicted to surround the Eta Chamaeleontis cluster will be one of the first goals of the project.

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arXiv2007

Measuring the Balmer Jump and Effective Gravity in FGK Stars

It is difficult to accurately measure the effective gravity (log g) in late-type stars using broadband (eg. UBV or SDSS) or intermediate-band (uvby) photometric systems, especially when the stars can cover a range of metallicities and reddenings. However, simple spectroscopic observational and data reduction techniques can yield accurate values for log g through comparison of the Balmer jumps of low-resolution spectra with recent grids of synthetic flux spectra.

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arXiv2006

Chemical Abundance Analysis of the Extremely Metal-Poor Star HE 1300+0157

We present a detailed chemical abundance analysis of HE 1300+0157, a subgiant with [Fe/H]=-3.9. From a high-resolution, high-S/N Subaru/HDS spectrum we find the star to be enriched in C ([C/Fe]_1D ~ +1.4) and O ([O/Fe]_1D ~ +1.8). With the exception of these species, HE 1300+0157 exhibits an elemental abundance pattern similar to that found in many other very and extremely metal-poor stars. The Li abundance is lower than the Spite-plateau value, in agreement with expectation for its evolutionary status. Of particular interest, no neutron-capture elements are detected in HE 1300+0157. This type of abundance pattern has been found by recent studies in several other metal-poor giants. We suggest that HE 1300+0157 is an unevolved example of this group of stars, which exhibit high C abundances together with low (or absent) abundances of neutron-capture elements. Several potential enrichment scenarios are presented. The low neutron-capture elements, including Sr, Ba, and Pb, suggests that the carbon excess observed in HE 1300+0157 is not due to mass transfer across a binary system. Such a scenario is applied to carbon-rich objects with excesses of s-process elements. The normal observed Li abundance supports this interpretation. Most likely, the high levels of C and O were produced prior to the birth of this star. It remains unclear whether a single hypernova is responsible for its overall chemical pattern, or whether one requires a superposition of yields from a massive Population III object and a normal Type II SN. These scenarios provide important information on the C production in the early Universe, and on the formation of C-rich stars in the early Galaxy.

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arXiv2005

The Metallicity Distribution Function of the Halo of the Milky Way

We report on the distribution of metallicities, [Fe/H], for very metal-poor stars in the halo of the Galaxy. Although the primary information on the nature of the Metallicity Distribution Function (MDF) is obtained from the two major recent surveys for metal-poor stars, the HK survey of Beers and collaborators, and the Hamburg/ESO Survey of Christlieb and collaborators, we also discuss the MDF derived from the publicly available database of stellar spectra and photometry contained in the third data release of the Sloan Digital Sky Survey (SDSS DR-3). Even though the SDSS was not originally planned as a stellar survey, significant numbers of stars have been observed to date -- DR-3 contains spectroscopy for over 70,000 stars, at least half of which are suitable for abundance determinations. There are as many very metal-poor ([Fe/H] < -2.0) stars in DR-3 as have been obtained from all previous survey efforts combined. We also discuss prospects for significant expansion of the list of metal-poor stars to be obtained from the recently funded extension of the SDSS, which includes the project SEGUE: Sloan Extension for Galactic Understanding and Exploration.

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arXiv2005

Kinematics of the Galactic Halo from Horizontal Branch stars in the Hamburg/ESO Survey

Large samples of Field Horizontal Branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ~7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially-dominated orbits to tangentially-dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic centre might help to differentiate them.

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arXiv2002

A stellar relic from the early Milky Way

The chemical composition of the most metal-deficient stars reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts; if totally metal-free (``population III'') stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun, leading to the suggestion that low-mass stars could only form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist, that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.

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arXiv1999

Catalogue of candidate emission-line objects in the Small Magellanic Cloud

Hαand [O III] narrow band, wide field (7 * 7 degree), CCD images of the Small Magellanic Cloud were compared and a catalogue of candidate planetary nebulae and Hαemission-line stars was compiled. The catalogue contains 131 planetary nebulae candidates, 23 of which are already known to be or are probable planetary nebulae or very low excitation objects. Also, 218 emission-line candidates have been identified with 113 already known. Our catalogue therefore provides a useful supplement to those of Meyssonnier & Azzopardi (1993) and Sanduleak, MacConnell & Davis Phillip (1978). Further observations are required to confirm the identity of the unknown objects.

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arXiv1998

Spectroscopy of Be stars in the Small Magellanic Cloud cluster NGC 330

The presence of an anomalously large population of Be stars in the young cluster NGC 330 in the SMC has been noted by Feast (1972), Bessell & Wood (1993), Grebel (1996) and most recently Keller et al. (1998). We present results from follow-up medium resolution spectra of the bright Be stars identified in Keller et al. (1998) and in the spectroscopic study of Mazzali et al. (1996). We find that the study of Mazzali et al. has overestimated the number of Be stars within NGC 330. Many of the bright B type stars identified by Mazzali et al. as Be stars show no signs of emission after correction for the diffuse HII emission pervading the cluster field. In the study of Mazzali et al. (1996) evidence is presented suggesting that all Be stars in NGC 330 have a common inclination of their rotation axes to the line of sight. For our sample of Be stars we present the emission equivalent widths and observed rotational velocities. An examination of these quantities shows that there is no evidence of a preferential alignment of the rotational axes, rather we are observing a population with random rotational axis alignment and disk size.

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