arXiv subjects

Ben Burningham

Publications and source records attributed to Ben Burningham.

58 records · Page 4Linked to original sources

arXiv2005

Can variability account for apparent age spreads in OB association colour-magnitude diagrams?

We have investigated the role of photometric variability in causing the apparent age spreads observed in the colour-magnitude diagrams of OB associations. We have found that the combination of binarity, photometric uncertainty and variability on timescales of a few years is not sufficient to explain the observed spread in either of the OB associations we have studied. Such effects can account for about half the observed spread in the sigma Orionis subgroup and about 1/20 of the observed spread in Cep OB3b. This rules out variability caused by stellar rotation and rotation of structures within inner accretion discs as the source of the majority of the the apparent age spreads. We also find that the variability tends to move objects parallel to isochrones in V/V-i' CMDs, and thus has little influence on apparent age spreads. We conclude that the remaining unexplained spread either reflects a true spread in the ages of the PMS objects or arises as a result of longer term variability associated with changes in accretion flow.

astro-ph↗

arXiv2005

Do accretion discs regulate the rotation of young stars?

We present a photometric study of I-band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars (M < 0.25 Msun) and classical T-Tauri stars. This increased sensitivity to periodicities is due to the enhanced depth and temporal density of our observations, compared with previous studies. The period distribution is at first glance similar to that seen in the Orion Nebula Cluster, with the higher-mass stars (M > 0.25 Msun) showing a bi-modal period distribution concentrated around periods of 2 and 8 days, and the lower-mass stars showing a uni-modal distribution, heavily biassed towards fast rotators. Closer inspection of the period distribution shows that the higher mass stars show a significant dearth of fast rotators, compared to the Orion Nebula Cluster, whilst the low mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K-L colour or H-alpha equivalent width. We also present a discussion of our own IC 348 data in the context of previously published period distributions for the Orion Nebula Cluster, the Orion Flanking Fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might in fact result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc locking paradigm, given the similarity in ages and disc fractions between the two clusters.

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arXiv2004

Contamination and exclusion in the sigma Orionis young group

We present radial velocities for 38 low-mass candidate members of the sigma Orionis young group. We have measured their radial velocities by cross-correlation of high resolution (R~6000) AF2/WYFFOS spectra of the gravity sensitive NaI doublet at 8183, 8195Angstroms. The total sample contained 117 objects of which 54 have sufficient signal-to-noise to detect NaI at an equivalent width of 3Angstroms, however we only detect NaI in 38 of these. This implies that very low-mass members of this young group display weaker NaI absorption than similarly aged objects in the Upper Scorpius OB association. We develop a technique to assess membership using radial velocities with a range of uncertainties that does not bias the selection when large uncertainties are present. The resulting membership probabilities are used to assess the issue of exclusion in photometric selections, and we find that very few members are likely to be excluded by such techniques. We also assess the level of contamination in the expected pre-main sequence region of colour-magnitude space brighter than I = 17. We find that contamination by non-members in the expected PMS region of the colour-magnitude diagram is small. We conclude that although radial velocity alone is insufficient to confirm membership, high signal-to-noise observations of the NaI doublet provide the opportunity to use the strength of NaI absorption in concert with radial velocities to asses membership down to the lowest masses, where Lithium absorption no longer distinguishes youth.

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arXiv2003

On the Nature of Collinder 121: Insights from the Low-Mass Pre-Main Sequence

We present a VI photometric catalogue towards the open cluster Cr 121. XMM-Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sight-line. de Zeeuw et al (1999) identified Cr 121 as a moving group, using HIPPARCOS data, at a distance of 592 pc. We reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sight-line, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050 pc. This is consistent with the original description of Cr 121 (Collinder, 1931), and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al (1999) resembles a foreground association and we agree with Eggen (1981) that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al (1999) census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.

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