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ABSTRACT: Context. The physical origin of the strong magnetic activity in T Tauri stars and its relation to stellar rotation is not yet well-understood. Aims. We investigate the relation between the X-ray activity, rotation, and Rossby number for a sample of young stars in the ≈ 3 Myr old cluster IC 348. Methods. We use the data of four Chandra observations of IC 348 to derive the X-ray luminosities of the young stars. Basic stellar parameters and rotation rates are collected from the literature. This results in a sample of 82 X-ray detected stars with known rotation periods. We determine the Rossby numbers (i.e. the ratio of rotation period to convective turnover time) of 76 of these stars from stellar structure-and evolution-models for pre-main sequence stars. Results. The young stars in IC 348 show no correlation between X-ray activity and rotation period. For the Rossby numbers, nearly all IC 348 stars are in the saturated regime of the activity–rotation relation defined by main-sequence stars. Searching for possible super-saturation effects, we find a marginal (but statistically in-significant) trend that the stars with the smallest Rossby numbers have slightly lower X-ray activity levels. There are no significant differences in the X-ray activity level for stars of different spectral types and no relation between spectral type and Rossby number is seen. In addition, for stars belonging to different IR-classes, no significant differences are present for the X-ray activity level as well as for their Rossby numbers. We compare the dispersion in the fractional X-ray luminosities of the stars in the saturated rotation regime in IC 348 to that seen in younger and older stellar populations. The scatter seen in the ≈ 3 Myr old IC 348 [σ log (L X /L bol) = 0.43] is considerably smaller than for the ≈ 1 Myr old Orion Nebula Cluster [ σ log(L X /L bol) = 0.63], but, at the same time, considerably larger than the dispersion seen in the ≈ 30 Myr old cluster NGC 2547 [σ log(L X /L bol) = 0.24] and in main-sequence stars. Conclusions. The results of our X-ray analysis of IC 348 show that neither the rotation rates nor the presence/absence of circum-stellar disks are of fundamental importance for determining the level of X-ray activity in TTS. Our results suggest that the scatter in the X-ray activity levels for the rapidly rotating members of young clusters decreases with the age of the stellar population. We interpret this as a signature of the changing interior structure of pre-main sequence stars and the consequent changes in the dynamo mechanisms that are responsible for the magnetic field generation.
03/2012;
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ABSTRACT: We study the properties of the coronae of the low-mass stars in the young
(~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and
optical/infrared data. The four existing Chandra observations of IC348 are
merged, thus providing a deeper and spatially more complete X-ray view than
previous X-ray studies of the cluster. We have compiled a comprehensive catalog
of IC348 members taking into account recent updates to the cluster census. Our
data collection comprises fundamental stellar parameters, infrared excess
indicating the presence of disks, Halpha emission as a tracer of chromospheric
emission or accretion and mass accretion rates. We have detected 290 X-ray
sources in four merged Chandra exposures, of which 187 are associated with
known cluster members. Only four of the X-ray sources are brown dwarfs
(spectral type M6 and later). The detection rate is highest for diskless Class
III stars and increases with stellar mass. This may be explained with higher
X-ray luminosities for higher mass and later evolutionary stage that is evident
in the X-ray luminosity functions. In particular, we find that for the lowest
examined masses (0.1-0.25 Msun) there is a difference between the X-ray
luminosity functions of accreting and non-accreting stars (classified on the
basis of their Halpha emission strength) as well as those of disk-bearing and
diskless stars (classified on the basis of the slope of the spectral energy
distribution). These differences disappear for higher masses. This is related
to our finding that the L_x/L_bol ratio is non-constant across the
mass/luminosity sequence of IC348 with a decrease towards lower luminosity
stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster
suggests that the decline of L_x/L_ bol for young stars at the low-mass end of
the stellar sequence is likely universal.
11/2011;
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ABSTRACT: The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the "GU CMa" cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to M_star ~0.5 M_sun, and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region. Comment: 19 pages, 15 figures, accepted by A&A
09/2009;
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K. V. Getman,
E. Flaccomio,
P. S. Broos,
N. Grosso,
M. Tsujimoto,
L. Townsley,
G. P. Garmire,
J. Kastner,
J. Li,
F. R. Harnden, [......],
M. J. McCaughrean,
G. Meeus,
F. Damiani,
G. Micela,
S. Sciortino,
J. Bally,
L. A. Hillenbrand,
W. Herbst, T. Preibisch,
and E. D. Feigelson
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ABSTRACT: We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep 2003 January Chandra X-Ray Observatory (Chandra) observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive data set on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.
The Astrophysical Journal Supplement Series 12/2008; 160(2):319. · 13.46 Impact Factor
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M. Guedel,
K. Briggs,
K. Arzner,
M. Audard,
J. Bouvier,
C. Dougados,
E. Feigelson,
E. Franciosini,
A. Glauser,
N. Grosso, [......],
D. Padgett,
F Palla,
I. Pillitteri, T. Preibisch,
L. Rebull,
L. Scelsi,
B. Silva,
S. Skinner,
B. Stelzer,
A. Telleschi
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ABSTRACT: We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the "XMM-Newton Extended Survey of the Taurus Molecular Cloud." X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a "soft excess" in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams. Comment: 8 pages, 9 figures, in press, Proc. IAUS 243, Star-Disk Interactions in Young Stars
Proceedings of the International Astronomical Union 11/2007;
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T. Preibisch,
M. ~J. McCaughrean,
N. Grosso,
E. ~D. Feigelson,
E. Flaccomio,
K. Getman,
L. ~A. Hillenbrand,
G. Meeus,
G. Micela,
S. Sciortino,
B. Stelzer
apjs. 10/2005; 160:582-593.
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T. Preibisch,
Y. -C. Kim,
F. Favata,
E. ~D. Feigelson,
E. Flaccomio,
K. Getman,
G. Micela,
S. Sciortino,
K. Stassun,
B. Stelzer,
H. Zinnecker
apjs. 10/2005; 160:401-422.
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ABSTRACT: A new proper motion study for about 1400 stars with $R < 18$ mag in a one square degree region around the young open cluster IC 348 based on Schmidt plates is presented. With an overall accuracy of about 3 mas/yr (2.5 mas/yr, respectively for all stars with $R < 17$ mag) we have obtained membership probabilities dividing our sample into three groups: foreground stars, cluster stars and background stars. This separation is also supported by the proper motion distribution with respect to the direction of the standard antapex and an increase of mean proper motion cluster membership probabilities with smaller cluster radii. 240 stars of our sample have cluster membership probabilities larger than 80%. The results are compared with the highly accurate proper motion study of [CITE]) and with other catalogues (PPM, ACT, Hipparcos), all containing only bright stars. From PPM and ACT proper motions a cluster radius of about 30 arcmin can be assumed. Hipparcos proper motions and parallaxes allow the separation of foreground stars, cluster stars and background stars in a somewhat larger region around IC 348 (with distances from the cluster centre of up to 85 arcmin). On the basis of Hipparcos data we calculate a mean distance of $261_{-23}^{+27}$ pc for the cluster stars with common proper motion. This is nearly the same distance as obtained by [CITE]) for the Per OB 2 association covering more than $15 \times 10$ square degrees. The mean proper motion of the cluster IC 348 obtained in our study is in good agreement with that of the highly probable members of the Per OB 2 association according to [CITE]). Therefore, we conclude that the cluster IC 348 is embedded in the Per OB 2 association. From our proper motion membership probabilities we found a cluster radius of $10-15$ arcmin. There is a concentration of Fredrick's highly probable cluster stars just in between our cluster stars and the distant field stars proper motion distributions. On the other hand, two groups of stars in the proper motion diagram of [CITE]) also seem to be indicated if all his possible cluster members are considered. The X-ray sources from [CITE]) identified with optical counterparts from our proper motion sample show a strong concentration in the proper motion diagram. For these objects which are likely T Tauri stars we obtained high membership probabilities so that we conclude that they belong to the cluster IC 348 and to the Per OB 2 association. The proper motion foreground stars and cluster members which are counterparts of near-infrared (NIR) sources [CITE]) are located in different parts of the $(J-H)-(H-K)$ two colour diagram, respectively.
Astronomy and Astrophysics Supplement Series 03/1999; 137:305.
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ABSTRACT: We have observed the young stellar cluster IC 348 in X-rays for the
first time. In very deep pointings with the ROSAT PSPC and HRI we have
detected a total of 116 X-ray sources. Most sources appear to be
embedded (A_V_=~5) in their parent molecular cloud. We use near-infrared
(J,H,K) photometric data from the study of Lada & Lada (1995) to
derive individual extinctions in order to calculate the X-ray
luminosities. We find 56 stars to be probable new cluster members,
presumably weak line T Tauri stars, because of their X-ray properties.
Since previously 49 Hα emission line stars and 17 proper motion
members were known, this nearly doubles the number of known member stars
in IC 348. The early type (B5-F5) stars do not appear to be intrinsic
X-ray emitters. The late type stars display similar X-ray properties as
those in other star forming regions. The weak line T Tauri stars seem to
be stronger X-ray emitters than the classical T Tauri stars, but this
difference may be caused by a selection effect. The total integrated
X-ray luminosity in IC 348 is at least 1.6x10^32^erg/s in the ROSAT band
(0.1-2.4keV). The possible influence of the X-radiation on the
ionization state of the cloud and on its further evolution is briefly
discussed. We report the detection of several large X-ray flares in IC
348 on timescales of typically a few hours. For some sources we also
find variability on time scales of several years by comparing their
count rates in the two pointed ROSAT observations and the ROSAT All Sky
Survey. About 45% of the X-ray sources show considerable variability on
long or short time scales.
Astronomy and Astrophysics 05/1996; 310:456-473. · 4.59 Impact Factor
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ABSTRACT: Herbig Ae/Be stars are young, pre-main-sequence stars of intermediate
mass (2-8 Msun). They are the more massive analogs to T Tauri
stars, but unlike T Tauri stars they are not believed to be convective,
convection being a pre-requisite for sustaining magnetic activity,
necessary for coronal heating and thus X-ray emission. The authors
present ROSAT observations of 30 Ae/Be stars, about half of which were
detected as X-ray sources with rather high X-ray luminosities up to
1032erg/s. The X-ray spectra and hardness ratios indicate
plasma temperatures of 107K, typical for coronal emission.
The high detection rate of X-ray emission from Ae/Be stars is
surprising, if the above reasoning is correct. The authors discuss
several possibilities for the origin of the X-ray emission: shock
interaction of the Ae/Be star stellar wind with remnant circumstellar
matter, coronal emission caused by a shear dynamo, and unresolved T
Tauri companions.
01/1996; -1:17-20.
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13:23.
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62:281.
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ABSTRACT: We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20 M.. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001) for low-mass pre-main- sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5-18.0 in a 6 deg(2) area. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg(2) and contains 166 new PMS stars. Combining these results with our earlier investigation ( Preibisch & Zinnecker) yields a sample of 250 PMS stars in the mass range similar to0.1 to similar to2 M. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low- mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 up to 20 M.. A power-law fit to the mass function gives a slope of alpha similar to -2.6 above similar to2 M. and a much flatter slope (alphasimilar to -0.9) below similar to0.6 M.. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered and support previous conjectures that the triggering event was a supernova shock wave originating from the nearby Upper Centaurus-Lupus association.
Astronomical Journal, v.124, 404-416 (2002).
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ABSTRACT: We explore the X-ray properties of the young stellar and substellar objects in the open cluster IC 348 as seen in our deep Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer image. First, we give identifications of all X-ray sources and determine upper limits for the X-ray luminosities of the undetected cluster members. Then we analyze the X-ray spectra of the young stellar objects, deriving plasma temperatures between similar to0.7 and similar to3 keV for the T Tauri stars in IC 348 and higher temperatures, between similar to3 and similar to7 keV, for flaring sources and two embedded young stellar objects. We find several large X-ray flares, in some of which a clear hardening of the X-ray spectra during the are peak is seen. Next we use the exceptional optical, infrared, and X-ray data set of this cluster to study various correlations and their implications, and to discuss new answers to some long-standing questions related to X-ray emission from young (sub) stellar objects. The X-ray luminosities of the young low-mass stars are strongly correlated to the stellar bolometric luminosities (L-X similar to 10(-4) x L-bol). Also, a good correlation between X-ray luminosity and stellar mass is found (L-X proportional to M-2). For the weak-line T Tauri stars we find a tight correlation between X-ray activity and chromospheric activity (L-X proportional to L-Halpha(0.8)), supporting the hypothesis that the chromosphere is heated by X-rays from the overlying corona. The observed X-ray properties of the brown dwarfs ( and brown dwarf candidates) are very similar to those of late-type stars; we explain this behavior as the consequence of the fact that very young substellar objects are still warm enough to maintain partially ionized atmospheres, which are capable of sustaining electrical currents, while in the cooler neutral atmospheres of L and T dwarfs such currents are shut off ( hence no X-ray emission). Finally, we explore the difference between the X-ray luminosity functions of classical and weak-line T Tauri stars. We find that the classical T Tauri stars in IC 348 seem to be on average less X-ray luminous than the weak-line T Tauri stars. However, we suggest that this apparent difference is caused by a selection effect: there is a strong detections bias against those weak-line T Tauri stars that are optically faint and hence X-ray faint; the population of classical T Tauri stars, on the other hand, is essentially completely known because of its very prominent Halpha emission. This conclusion is corroborated by another new result: when using a photometrically selected, magnitude-limited, complete sample of T Tauri stars and taking the K L infrared excess as a tracer of circumstellar material, we find no evidence in IC 348 for a difference in X-ray properties of young stars with and without circumstellar matter, i.e., classical and naked T Tauri stars.
Astronomical Journal, v.123, 1613-1628 (2002).
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Grebel, Eva K.; Brandner, Wolfgang: Modes of Star Formation and the Origin of Field Populations, Astronomical Society of the Pacific, 97-104 (2002).
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Alves, Joao F.; McCaughrean Mark J.: The Origins of Stars and Planets : the VLT View, Springer (2002).
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Favata, F.; Drake, J.: Stellar Coronae in the Chandra and XMM-Newton Era, Astronomical Society of the Pacific, 185-192 (2002).
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ABSTRACT: Aims. The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays.Methods. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources.Results. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the “GU CMa” cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to $M_\star$ ~ 0.5 $M_\odot$, and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region.
http://dx.doi.org/10.1051/0004-6361/200912140.
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ABSTRACT: We analyze a XMM-Newton X-ray imaging observation of the very young stellar cluster IC 348 with an observing time of 12 h and a corresponding total (all detector) MOS-equivalent exposure time of 207 ks. Our observation is strongly affected by a very intense solar particle flare, due to which only the first half of the exposure time can be used for scientific analysis. We compare the X-ray sources seen by XMM-Newton with those found in our previous Chandra study of IC 348 and find that XMM-Newton reveals 71 new X-ray sources, most of which are located outside the field-of-view of the Chandra observation. 20 of these new X-ray sources can be identified with known cluster members, and 19 sources are likely to be new low-mass members of the cluster. The lightcurves of the XMM-Newton sources show at most moderate levels of variability, but no large flares. We compare the spectral fitting results for 10 stars for which we have good spectra from both Chandra and XMM-Newton, and find good agreement in the fitted plasma temperatures, but a systematic difference in the fitted values of the hydrogen column density (extinction). We finally discuss the X-ray properties of the optically bright B1 star $o$ Per and the X-ray detections among the deeply embedded young stellar objects to the south of the optical cluster center (including HH 211-mm), and find that none of the 23 spectroscopically identified brown dwarfs in IC 348 is detected in our XMM-Newton data.
http://dx.doi.org/10.1051/0004-6361:20040214.
