arXiv subjects

Michael S. Bessell

Publications and source records attributed to Michael S. Bessell.

At least 37 records · Page 2Linked to original sources

arXiv2014

Reddening, Distance, and Stellar Content of the Young Open Cluster Westerlund 2

We present deep $UBVI_C$ photometric data of the young open cluster Westerlund 2. An abnormal reddening law of $R_{V,cl}=4.14\pm0.08$ was found for the highly reddened early-type members ($E(B-V)\geq 1.45$), whereas a fairly normal reddening law of $R_{V,fg}=3.33\pm0.03$ was confirmed for the foreground early-type stars ($E(B-V)_{fg}<1.05$). The distance modulus was determined from zero-age main-sequence (ZAMS) fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be $V_0-M_V=13.9\pm0.14$ (random error) $_{-0.1}^{+0.4}$ (the upper limit of systematic error) mag ($d = 6.0 \pm 0.4 _{-0.3}^{+1.2}$ kpc). To obtain the initial mass function, pre-main-sequence (PMS) stars were selected by identifying the optical counterparts of Chandra X-ray sources and mid-infrared emission stars from the Spitzer GLIMPSE source catalog. The initial mass function shows a shallow slope of $Γ=-1.1 \pm 0.1$ down to $\log m = 0.7$. The total mass of Westerlund 2 is estimated to be at least 7,400 $M_{\odot}$. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be $\lesssim$ 1.5 Myr. We confirmed the existence of a clump of PMS stars located $\sim1$ arcmin north of the core of Westerlund 2, but we could not find any clear evidence for an age difference between the core and the northern clump.

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arXiv2014

Sejong Open Cluster Survey (SOS). III. The Young Open Cluster NGC 1893 in the H II Region W8

We present a UBV I and H alpha photometric study of the young open cluster NGC 1893 in the H II region W8 (IC 410 or Sh 2-236). A total of 65 early-type members are selected from photometric diagrams. A mean reddening of the stars is = 0.563 +/- 0.083 mag. The published photometric data in the near- and mid-infrared passbands are used to test the reddening law toward the cluster, and we confirm that the reddening law is normal (R_V = 3.1). Zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 12.7 +/- 0.2 mag, equivalent to 3.5 +/- 0.3 kpc. From H alpha photometry 125 H alpha emission stars and candidates are identified as pre-main sequence (PMS). The lists of young stellar objects and X-ray sources published by previous studies allow us to select a large number of PMS members down to 1 M_sun. Isochrone fitting in the Hertzsprung-Russell diagram gives a turn-off age of 1.5 Myr and the median age of 1.9 Myr from the PMS members with a spread of 5 Myr. We derive the initial mass function (IMF) for stars with mass larger than 1 M_sun. The slope of the IMF (Gamma = -1.3 +/- 0.1) is well consistent with the Salpeter/Kroupa IMF. A total mass of the cluster appears to be in excess of 1,300 M_sun. Finally, we estimate the mass accretion rate of 82 PMS members in the mass range of 0.6 M_sun to 5 M_sun.

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arXiv2013

Sejong Open Cluster Survey (SOS). II. IC 1848 Cluster in the H II Region W5 West

IC 1848 is one of the young open clusters in the giant star forming Cas OB6 association. Several interesting aspects relating to star formation processes in giant star forming regions attracted us to study the initial mass function (IMF), star formation mode, and properties of pre-main sequence stars (PMS). A UBVI and H alpha photometric study of the young open cluster IC 1848 was conducted as part of the "Sejong Open cluster Survey" (SOS). We have selected 105 early-type members from photometric diagrams. Their mean reddening is = 0.660 +/- 0.054 mag. Using the published photometric data with near- and mid-infrared archival data we confirmed the normal reddening law (R_V = 3.1) toward the cluster (IC 1848). A careful zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 11.7 +/- 0.2 mag, equivalent to 2.2 +/- 0.2 kpc. H alpha photometry and the list of young stellar objects identified by Koenig et al. permitted us to select a large number of PMS stars comprising 196 H alpha emission stars, 35 H alpha emission candidates, 5 Class I, 368 Class II, and 24 transition disk candidates. From the Hertzsprung-Russell diagram using stellar evolution models, we estimate an age of 5 Myr from several evolved stars and 3 Myr from the PMS stars. The IMF was derived from stars with mass larger than 3 M_sun, and the slope is slightly steeper (Gamma = -1.6 +/- 0.2) than the Salpeter/Kroupa IMF. Finally, we estimated the mass accretion rate of PMS stars with a UV excess. The mean mass accretion rate is about 1.4 x 10^-8 M_sun yr^-1 in the mass range of 0.5 M_sun to 2 M_sun, whereas intermediate-mass stars (>= 2.5 M_sun) exhibit a much higher accretion rate of dM/dt > 10^-6 M_sun yr^-1.

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arXiv2013

Copious Amounts of Hot and Cold Dust Orbiting the Main Sequence A-type Stars HD 131488 and HD 121191

We report two new dramatically dusty main sequence stars: HD 131488 (A1V) and HD 121191 (A8V). HD 131488 is found to have substantial amounts of dust in its terrestrial planet zone (L_IR/L_bol~4x10^-3), cooler dust further out in its planetary system, and an unusual mid-infrared spectral feature. HD 121191 shows terrestrial planet zone dust (L_IR/L_bol~2.3x10^-3), hints of cooler dust, and shares the unusual mid-infrared spectral shape identified in HD 131488. These two stars belong to sub-groups of the Scorpius-Centaurus OB association and have ages of ~10 Myr. HD 131488 and HD 121191 are the dustiest main sequence A-type stars currently known. Early-type stars that host substantial inner planetary system dust are thus far found only within the age range of 5-20 Myr.

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arXiv2013

Re-examining the membership and origin of the Epsilon Cha association

We present a comprehensive investigation of the Epsilon Chamaeleontis association (Epsilon Cha), one of several young moving groups spread across the southern sky. We re-assess the putative membership of Epsilon Cha using the best-available proper motion and spectroscopic measurements, including new ANU 2.3-m/WiFeS observations. After applying a kinematic analysis our final membership comprises 35-41 stars from B9 to mid-M spectral types, with a mean distance of 110+/-7 pc and a mean space motion of (U,V,W)=(-10.9+/-0.8,-20.4+/-1.3,-9.9+/-1.4) km/s. Theoretical evolutionary models suggest Epsilon Cha is 3-5 Myr old, distinguishing it as the youngest moving group in the solar neighbourhood. Fifteen members show 3-22 micron spectral energy distributions attributable to circumstellar discs, including 11 stars which appear to be actively accreting. Epsilon Cha's disc and accretion fractions (29+8-6 and 32+9-7 per cent, respectively) are both consistent with a typical 3-5 Myr-old population. Multi-epoch spectroscopy reveals three M-type members with broad and highly-variable H-alpha emission as well as several new spectroscopic binaries. We reject 11 stars proposed as members in the literature and suggest they may belong to the background Cha I and II clouds or other nearby young groups. Our analysis underscores the importance of a holistic and conservative approach to assigning young stars to kinematic groups, many of which have only subtly different properties and ill-defined memberships. We conclude with a brief discussion of Epsilon Cha's connection to the young open cluster Eta Cha and the Sco-Cen OB association. Contrary to earlier studies which assumed Eta and Epsilon Cha are coeval and were born in the same location, we find the groups were separated by ~30 pc when Eta Cha formed 4-8 Myr ago in the outskirts of Sco-Cen, 1-3 Myr before the majority of Epsilon Cha members.

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arXiv2013

Sejong Open Cluster Survey (SOS). 0. Target Selection and Data Analysis

Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We have initiated the Sejong Open cluster Survey (SOS) - a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousins' $UBVI$ system. To achieve our main goal, we have paid much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small, sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp - Mv relations, Sp - Teff relations, Sp - color relations, and Teff - BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.

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arXiv2012

The Starburst Cluster Westerlund 1: The Initial Mass Function and Mass Segregation

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVIc and JKs photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral type - intrinsic color relation, we obtain the mean interstellar reddening of =4.19+/-0.23 and =1.70+/-0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster -- 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0+/-1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of Γ = -0.8 +/- 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 x 10^4 solar mass. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low-luminosity yellow hypergiants.

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arXiv2012

A High-Resolution Spectroscopic Search for the Remaining Donor for Tycho's Supernova

In this paper, we report on our analysis using Hubble Space Telescope astrometry and Keck-I HIRES spectroscopy of the central six stars of Tycho's supernova remnant (SN 1572). With these data, we measured the proper motions, radial velocities, rotational velocities, and chemical abundances of these objects. Regarding the chemical abundances, we do not confirm the unusu- ally high [Ni/Fe] ratio previously reported for Tycho-G. Rather, we find that for all metrics in all stars, none exhibit the characteristics expected from traditional SN Ia single-degenerate-scenario calculations. The only possible exception is Tycho-B, a rare, metal-poor A-type star; however, we are unable to find a suitable scenario for it. Thus, we suggest that SN 1572 cannot be explained by the standard single-degenerate model.

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arXiv2012

Hunting for the progenitor of SN 1006: High resolution spectroscopic search with the FLAMES instrument

Type Ia supernovae play a significant role in the evolution of the Universe and have a wide range of applications. It is widely believed that these events are the thermonuclear explosions of carbon-oxygen white dwarfs close to the Chandrasekhar mass (1.38 M\odot). However, CO white dwarfs are born with masses much below the Chandrasekhar limit and thus require mass accretion to become Type Ia supernovae. There are two main scenarios for accretion. First, the merger of two white dwarfs and, second, a stable mass accretion from a companion star. According to predictions, this companion star (also referred to as donor star) survives the explosion and thus should be visible in the center of Type Ia remnants. In this paper we scrutinize the central stars (79 in total) of the SN 1006 remnant to search for the surviving donor star as predicted by this scenario. We find no star consistent with the traditional accretion scenario in SN1006.

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arXiv2012

Rapid disappearance of a warm, dusty circumstellar disk

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution is typically ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

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arXiv2012

RX J0942.7-7726AB: an isolated pre-main sequence wide binary

We report the discovery of two young M-dwarfs, RX J0942.7-7726 (M1) and 2MASS J09424157-7727130 (M4.5), that were found only 42 arcsec apart in a survey for pre-main sequence stars surrounding the open cluster eta Chamaeleontis. Both stars have congruent proper motions and near-infrared photometry. Medium-resolution spectroscopy reveals that they are coeval (age 8-12 Myr), codistant (100-150 pc) and thus almost certainly form a true wide binary with a projected separation of 4000-6000 AU. The system appears too old and dynamically fragile to have originated in eta Cha and a traceback analysis argues for its birth in or near the Scorpius-Centaurus OB Association. RX J0942.7-7726AB joins a growing group of wide binaries kinematically linked to Sco-Cen, suggesting that such fragile systems can survive the turbulent environment of their natal molecular clouds while still being dispersed with large velocities. Conversely, the small radial velocity difference between the stars (2.7 \pm 1.0 km/s) could mean the system is unbound, a result of the coincidental ejection of two single stars with similar velocity vectors from the OB association early in its evolution.

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arXiv2012

The VAST Survey -- II. Orbital motion monitoring of A-type star multiples

As a part of our ongoing Volume-limited A-Star (VAST) adaptive optics survey, we have obtained observations of 26 binary systems with projected separations <100 AU, 13 of which have sufficient historical measurements to allow for refinement of their orbital elements. For each system with an estimated orbit, the dynamical system mass obtained was compared with the system mass estimated from mass-magnitude relations. Discrepancies between the dynamical and theoretical system mass can be explained by the presence of a previously unresolved spectroscopic component, or by a non-solar metallicity of the system. Using this approach to infer the presence of additional companions, a lower limit to the fraction of binaries, triples, and quadruples can be estimated as 39, 46, and 15 per cent, for systems with at least one companion within 100 AU. The fraction of multiple systems with three or more components shows a relative increase compared to the fraction for Solar-type primaries resolved in previous volume-limited surveys. The observations have also revealed a pair of potentially young ($<$100 Myr) M-dwarf companions, which would make an ideal benchmark for the theoretical models during the pre-Main Sequence contraction phase for M-dwarfs. In addition to those systems with orbit fits, we report 13 systems for which further orbital monitoring observations are required, 11 of which are newly resolved as a part of the VAST survey.

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arXiv2012

Distance and the Initial Mass Function of Young Open Clusters In The Eta Carina Nebula: Tr 14 and Tr 16

We present new UBVI_C CCD photometry of the young open clusters Trumpler 14 (Tr 14) and Trumpler 16 (Tr 16) in the eta Carina nebula. We also identify the optical counterpart of Chandra X-ray sources and Two Micron All Sky Survey point sources. The members of the clusters were selected from the proper motion study, spectral types, reddening characteristics, and X-ray or near-IR excess emission. An abnormal reddening law R_V,cl=4.4\pm0.2 was obtained for the stars in the eta Carina nebula using the 141 early-type stars with high proper motion membership probability (P_u\geq70). We determined the distance to each cluster and conclude that Tr 14 and Tr 16 have practically the same distance modulus of V_0-M_V=12.3\pm0.2 mag (d=2.9\pm0.3 kpc). The slope of the initial mass function was determined to be Γ=-1.3\pm0.1 for Tr 14, Γ=-1.3\pm0.1 for Tr 16, and Γ=-1.4\pm0.1 for all members in the observed region for the stars with log m\geq0.2. We also estimated the age of the clusters to be about 1 -- 3 Myr from the evolutionary stage of evolved stars and low-mass pre-main-sequence stars.

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arXiv2011

Spectrophotometric Libraries, Revised Photonic Passbands and Zero-points for UBVRI, Hipparcos and Tycho Photometry

We have calculated improved photonic passbands for the UBV RI, Hipparcos and Tycho Hp,BT,VT standard systems using the extensive spectrophotometric libraries of NGSL and MILES. Using the Hp passband, we adjusted the absolute flux levels of stars in the spectrophotometric libraries so their synthetic Hp magnitudes matched the precise Hipparcos catalog value. Synthetic photometry based on the renormalized fluxes were compared to the standard UBVRI and BT, VT magnitudes and revised synthetic zero-points were determined. The Hipparcos and Tycho photometry system zero-points were also compared to the V magnitude zero-points of the SAAO UBVRI system, the homogenized UBV system and the Walraven V B system. The confusion in the literature concerning broadband magnitudes, fluxes, passbands and the choice of appropriate mean wavelengths is detailed and discussed in an appendix.

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arXiv2011

Photonic Passbands and Zeropoints for the Stromgren uvby system

Photonic passbands have been derived for the uvby standard system by convolving the original filter passbands of Strömgren and Perry with atmospheric extinction and the QE of a cooled 1P21 photomultiplier tube. Using these new passbands, synthetic photometry was calculated for all the stars in the extensive NGSL and MILES spectrophotometric libraries and compared with the homogenised b-y, m1 and c1 indices in the Hauck-Mermilliod 1998 catalog and the derived u - v and v - b colors. Excellent agreement between observed and synthetic photometry was achieved with regression slopes near unity. Slightly better fits were obtained by considering stars with b-y < 0.5 and b-y > 0.5, separately. It is recommended that these new passbands be used together with the provided transformation equations to generate synthetic photometry from model atmosphere fluxes and observed spectrophotometry. Synthetic photometry was also carried out using the natural system of the 4-channel spectrograph-photometers and those of Cousins and Eggen in order to explore the systematic dfferences that could be expected between their instrumental systems and the standard system.

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arXiv2011

The Extragalactic Distance Scale without Cepheids IV

The Cepheid period-luminosity relation is the primary distance indicator used in most determinations of the Hubble constant. The tip of the red giant branch (TRGB) is an alternative basis. Using the new ANU SkyMapper Telescope, we calibrate the Tully Fisher relation in the I band. We find that the TRGB and Cepheid distance scales are consistent.

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arXiv2011

IRAS 15099-5856: Remarkable Mid-Infrared Source with Prominent Crystalline Silicate Emission Embedded in the Supernova Remnant MSH15-52

We report new mid-infrared observations of the remarkable object IRAS 15099-5856 using the space telescopes AKARI and Spitzer, which demonstrate the presence of prominent crystalline silicate emission in this bright source. IRAS 15099-5856 has a complex morphology with a bright central compact source (IRS1) surrounded by knots, spurs, and several extended (~4') arc-like filaments. The source is seen only at >= 10 um. The Spitzer MIR spectrum of IRS1 shows prominent emission features from Mg-rich crystalline silicates, strong [Ne II] 12.81 um and several other faint ionic lines. We model the MIR spectrum as thermal emission from dust and compare with the Herbig Be star HD 100546 and the luminous blue variable R71, which show very similar MIR spectra. Molecular line observations reveal two molecular clouds around the source, but no associated dense molecular cores. We suggest that IRS1 is heated by UV radiation from the adjacent O star Muzzio 10 and that its crystalline silicates most likely originated in a mass outflow from the progenitor of the supernova remnant (SNR) MSH 15-52. IRS1, which is embedded in the SNR, could have been shielded from the SN blast wave if the progenitor was in a close binary system with Muzzio 10. If MSH15-52 is a remnant of Type Ib/c supernova (SN Ib/c), as has been previously proposed, this would confirm the binary model for SN Ib/c. IRS1 and the associated structures may be the relics of massive star death, as shaped by the supernova explosion, the pulsar wind and the intense ionizing radiation of the embedded O star.

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arXiv2010

Revealing the Chamaeleon: 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. We propose 4 new probable cluster members and 3 possible members requiring further investigation. Candidates were selected on the basis of DENIS and 2MASS photometry, NOMAD astrometry and extensive follow-up spectroscopy. Several of these stars show substantial variation in their H-alpha emission line strengths on timescales of days to months, with at least one event attributable to accretion from a circumstellar disk. These findings are consistent with a dynamical origin for the current configuration of the cluster, without the need to invoke an abnormally top-heavy Initial Mass Function, as proposed by some authors.

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