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Title: Wide sigma Orionis binaries resolved by UKIDSS
Author: J. A. Caballero, I. Novalbos, T. Tobal, F. X. Miret

In spite of its importance for the study of star formation at all mass domains, the nearby young sigma Orionis cluster still lacks a comprehensive survey for multiplicity. We try to fill that observational gap by looking for wide resolved binaries with angular separations between 0.4 and 4.0 arcsec. We search for companions to 331 catalogued cluster stellar members and candidates in public K-band UKIDSS images outside the innermost 1 arcmin, which is affected by the glare of the bright, eponymous sigma Ori multiple system, and investigate their cluster membership with colour-magnitude diagrams and previous knowledge of youth features. Of the 18 identified pairs, ten have very low individual probabilities of chance alignment (< 1 %) and are considered here as physical pairs. Four of them are new, while the other six had been discovered previously, but never investigated homogeneously and in detail. Projected physical separations and magnitude differences of the ten probably bound pairs range from 180 to 1220 au, and from 0.0 to 3.4 mag in K, respectively. Besides, we identify two cluster stars with elongated point spread functions. We determine the minimum frequency of wide multiplicity in the interval of projected physical separations s = 160-1600 au in sigma Orionis at 3.0^{+1.2}_{-1.1} %. We discover a new Lindroos system, find that massive and X-ray stars tend to be in pairs or trios, conclude that multiplicity truncates circumstellar discs and enhances X-ray emission, and ascribe a reported lithium depletion in a young star to unresolved binarity in spectra of moderate resolution. When accounting for all know multiples, including spectroscopic binaries, the minimum frequency of multiplicity increases to about 10 %, which implies that of the order of 80-100 unknown multiple systems still await discovery in sigma Orionis.

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Sigma Orionis
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The characteristics of the multiple star "sigma Orionis" are determined

Some three million years ago hundreds of stars formed from a dense cloud of gas and dust in the constellation of Orion ("the Hunter"). The star which swallowed the largest part of the mass was sigma Orionis (sigma Ori), which is now the fourth brightest star in the belt of Orion, and which illuminates the famous Horsehead Nebula. At the same time as sigma Orionis, a large number of stars with a full range of masses formed in its neighbourhood, as well as brown dwarfs, and "rogue" planets (objects similar in mass to that of a Jupiter but which do not rotate around a star, rather they move freely in the star cluster). The smallest objects in Orion´s belt have 10,000 times less mass than sigma Orionis.
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Posts: 129897
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RE: Sigma Orionis cluster
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Title: A spectroscopic census in young stellar regions: the Sigma Orionis cluster
Author: Jesús Hernández, Nuria Calvet, Alice Perez, Cesar Briceño, Lorenzo Olguin, Maria E. Contreras, Lee Hartmann, Lori Allen, Catherine Espaillat, Ramírez Hernan

We present a spectroscopic survey of the stellar population of the Sigma Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than a half of stars of our sample are members confirmed by the presence of lithium in absorption, strong H in emission or weak gravity-sensitive features. In addition, we have obtained high resolution (R~34000) spectra in the Halpha region for 169 stars in the region. Radial velocities were calculated from this data set. The radial velocity distribution for members of the cluster is in agreement with previous work. Analysis of the profile of the Halpha line and infrared observations reveals two binary systems or fast rotators that mimic the Halpha width expected in stars with accretion disks. On the other hand there are stars with optically thick disks and narrow Halpha profile not expected in stars with accretion disks. This contribution constitutes the largest homogeneous spectroscopic data set of the Sigma Orionis cluster to date.

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Sigma Orionis E
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Title: MOST Observations of sigma Ori E: Challenging the Centrifugal Breakout Narrative
Authors: R. H. D. Townsend, Th. Rivinius, J. F. Rowe, A. F. J. Moffat, J. M. Matthews, D. Bohlender, C. Neiner, J. H. Telting, D. B. Guenther, T. Kallinger, R. Kuschnig, S. M. Rucinski, D. Sasselov, W. W. Weiss

We present results from three weeks' photometric monitoring of the magnetic helium-strong star sigma Ori E using the MOST microsatellite. The star's light curve is dominated by twice-per-rotation eclipse-like dimmings arising when magnetospheric clouds transit across and occult the stellar disk. However, no evidence is found for any abrupt centrifugal breakout of plasma from the magnetosphere, either in the residual flux or in the depths of the light minima. Motivated by this finding we compare the observationally inferred magnetospheric mass against that predicted by a breakout analysis. The large discrepancy between the values leads us to argue that centrifugal breakout does not play a significant role in establishing the magnetospheric mass budget of sigma Ori E.

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Title: Searching for CU Vir-type cyclotron maser from Sigma Ori E: The role of the magnetic quadrupole component
Authors: P. Leto, C. Trigilio, C. S. Buemi, F. Leone, G. Umana

In this paper we present new and archive radio measurements obtained with the Very Large Array of the magnetic chemically peculiar (MCP) star \sigma Ori E. The radio data have been obtained at different frequencies and are well distributed along the rotational phases. We analyse in detail the radio emission from \sigma Ori E with the aim to search evidence of circularly polarized radio pulses. Up to now, among the MCP stars only CU Virginis shows 100% polarised time-stable radio pulses, explained as highly directive electron cyclotron maser emission, visible from Earth at particular rotational phases, like a pulsar. Our analysis shows that there is no hint of coherent emission at frequencies below 15 GHz. We conclude that the presence of a quadrupolar component of the magnetic field, dominant within few stellar radii from the star, where the maser emission should be generated, inhibits the onset of the cyclotron maser instability in \sigma Ori E.

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RE: Sigma Orionis cluster
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Title: Gaia and sigma Orionis from 20 Msol to 3 MJup: the most complete and precise Initial Mass Function with a parallax determination?
Authors: Jose A. Caballero

The sigma Orionis cluster is to date the star-forming region with the largest number of confirmed brown dwarfs and substellar objects below the deuterium burning mass limit. The most massive star, sigma Ori Aa, just in the cluster centre, is the ~20Msol-mass O9.5V star that illuminates the Horsehead Nebula, while the least massive object yet reported, S Ori 70, is only around 3 MJup. In the middle, there is a continuum of stars and substellar objects of all types (including magnetically active B2Vp stars, Herbig-Haro objects, FU Ori stars or T Tauri brown dwarfs) that makes the cluster a cornerstone in the study of the initial mass function, disc presence, X-ray emission or accretion at all mass domains. However, the derived masses strongly depend on the actual heliocentric distance to the cluster. Gaia will solve the dilemma.

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Sigma Orionis AB system
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Title: A third massive star component in the sigma Orionis AB system
Authors: S. Simón-Díaz, J.A. Caballero, J. Lorenzo

We report on the detection of a third massive star component in the sigma Orionis AB system, traditionally considered as a binary system. The system has been monitored by the IACOB spectroscopic survey of Northern Massive stars program, obtaining 23 high-resolution FIES@NOT spectra with a time-span of ~2.5 years. The analysis of the radial velocity curves of the two spectroscopic components observed in the spectra has allowed us obtain the orbital parameters of the system, resulting in a high eccentric orbit (e~0.78) with an orbital period of 143.5 ±0.5 d. This result implies the actual presence of three stars in the sigma Orionis AB system when combined with previous results obtained from the study of the astrometric orbit (with an estimated period of ~157 yr).

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Sigma Orionis
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Title: Discovery of an old photoevaporating disk in sigma Orionis
Authors: E. Rigliaco, A. Natta, S. Randich, G. Sacco

The photoevaporation of circumstellar disks is a powerful process in the disk dissipation at the origin of the Orion proplyds. This Letter reports the first detection of a photoevaporating disk in the final but long-lasting phase of its evolution. The disk is associated to a low-mass T Tauri member of the sigma Orionis Cluster. It is characterised by a very low (if any) accretion rate and by a tenuous (Mloss ~ 10^{-9} Msun/yr) photoevaporation wind, which is unambiguously detected in the optical spectrum of the object. The wind emits strong forbidden lines of [SII] and [NII] because the low-mass star is close to a powerful source of ionising photons, the O9.5 star sigma Ori.

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Perhaps the finest multiple star in the sky visible to both northern and southern observers, sigma Orionis is a system of five stars, four of which are visible in a small telescope.  The brightest star of this group is one of the most luminous known and will one day expire, like many stars in Orion, in a spectacular supernova explosion.

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Sigma Orionis or Sigma Ori is a five star system in the constellation Orion. It is approximately 1,150 light years from Earth.

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Position(200): RA 05 38 44.7680, Dec  -02 36 00.248

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Posts: 129897
Date:
Sigma Orionis cluster
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Title: Stars and brown dwarfs in the sigma Orionis cluster: the Mayrit catalogue
Authors: Jose A. Caballero

The young sigma Orionis cluster is an indispensable basis for understanding the formation and evolution of stars, brown dwarfs and planetary-mass objects. Our knowledge of its stellar population is, however, incomplete. I present the Mayrit catalogue, that comprises most of the stars and high-mass brown dwarfs of the cluster. The basis of this work is an optical-near infrared correlation between the 2MASS and DENIS catalogues in a circular area of radius 30 arcmin centred on the OB-type binary sigma Ori AB. The analysis is supported on a bibliographic search of confirmed cluster members with features of youth and on additional X-ray, mid-infrared and astrometric data. I list 241 sigma Orionis stars and brown dwarfs with known features of youth, 97 candidate cluster members (40 are new) and 115 back- and foreground sources in the survey area. The 338 cluster members and member candidates constitute the Mayrit catalogue. This catalogue is a suitable input for studying the spatial distribution, multiplicity, properties and frequency of discs and the complete mass function of sigma Orionis.

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