arXiv subjects

Nicolas Grosso

Publications and source records attributed to Nicolas Grosso.

41 records · Page 3Linked to original sources

arXiv2001

Discovery of new embedded Herbig-Haro objects in the rho Ophiuchi dark cloud

We report here the discovery of a 30"-chain of embedded Herbig-Haro (HH) objects in the rho Ophiuchi dark cloud. These HH objects were first detected during a deep K_S-band observation (completeness magnitude for point source~19) made with NTT/SOFI. We confirm their nature with follow-up observations made with H_2 v=1-0 S(1) narrow-band filter. We argue that they belong to two different jets emanating from two Class I protostars: the main component of the recently resolved subarcsecond radio binary YLW15 (also called IRS43), and IRS54. We propose also to identify the [S II] knot HH224NW1 (Gomez et al 1998) as emanating from a counterjet of YLW15. The alignment between these HH objects and the thermal jet candidate found in YLW15 by Girart et al. (2000) implies that this jet is not precessing at least on timescale ~(2-4)x1E4 yr.

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arXiv2001

ROSAT-HRI detection of the Class I protostar YLW16A in the rho Ophiuchi dark cloud

I analyze unpublished or partially published archival ROSAT data of the rho Ophiuchi dark cloud. This set of seven overlapping ROSAT HRI pointings, composed of eight ~one-hour exposures, detects mainly the X-ray brightest T Tauri stars of this star-forming region. Only two HRI sources are new X-ray sources, and their optical counterparts are proposed as new Weak T Tauri star candidates. Meanwhile the ROSAT HRI caught during just one exposure a weak X-ray source (maximum likelihood=10; SNR=4.1σfor Gaussian statistics) among a group of three embedded young stellar objects including two Class I protostars. Previous ROSAT PSPC, ASCA GIS observations, and as I argue here one Einstein IPC observation, have already detected an X-ray source in this area, but this higher angular resolution data show clearly that X-rays are emitted by the Class I protostar YLW16A. This is the second Class I protostar detected by the ROSAT HRI in this dark cloud. The determination of the intrinsic X-ray luminosity of this event, L_X[0.1-2.4 keV]=(9.4-450)*1E30 erg/s, critically depends on the source absorption estimate. Improvements will be obtained only by the direct determination of this parameter from fitting of Chandra and XMM-Newton spectra.

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arXiv2000

X-rays and regions of star formation: a combined ROSAT-HRI/near-to-mid IR study of the rho Oph dark cloud

We have obtained two deep exposures of the rho Oph cloud core region with the ROSAT HRI. The improved position accuracy (1"-6") with respect to previous recent X-ray observations (ROSAT PSPC, and ASCA) allows us to remove positional ambiguities for the detected sources. We also cross-correlate the X-ray positions with IR sources found in the ISO-ISOCAM survey of the same region at 6.7 and 14.3 \mum, in addition to sources known from ground-based observations, which are young stars. We find that there is no statistically significant difference between the X-ray luminosity functions of HRI-detected Class II and Class III sources, i.e., T Tauri stars with and without disks, confirming that the contribution of these disks to X-ray emission or absorption, must be small. Most of the sources are variable, and their variability is consistent with a solar-like (hence magnetic) flare origin. We use the information given both by the ISOCAM survey and by our HRI deep exposure to study the T Tauri star population of the rho Oph dense cores. We confirm that essentially all Class II and Class III sources are X-ray emitters, and that a strong correlation exists between their X-ray luminosity, L_X, and their stellar luminosity, L_*, with L_X/L_* ~ 1E-4. Most of the new ISOCAM Class II sources are not detected, however, which we explain by the fact that their X-ray luminosities ``predicted'' on the basis of this correlation are too faint to be detected by the HRI. We predict that ~40 unknown faint or embedded Class III sources remain to be discovered in X-rays in the HRI/ISOCAM overlapping area, down to a limit of L_X ~ 3 \times 1E28 erg/s. We show that the bulk of these unknown Class III sources should be made of low- to very low-mass stars. Prospects for future detections with XMM-Newton and Chandra are discussed.

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arXiv1999

Quasi-periodic X-ray Flares from the Protostar YLW15

With ASCA, we have detected three X-ray flares from the Class I protostar YLW15. The flares occurred every ~20 hours and showed an exponential decay with time constant 30-60 ks. The X-ray spectra are explained by a thin thermal plasma emission. The plasma temperature shows a fast-rise and slow-decay for each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows this time profile only for the first flare, and remains almost constant during the second and third flares at the level of the tail of the first flare. The peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are among the brightest X-ray luminosities observed to date for Class I protostars. The total energy released in each flare was 3-6*10^{36} ergs. The first flare is well reproduced by the quasi-static cooling model, which is based on solar flares, and it suggests that the plasma cools mainly radiatively, confined by a semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio \~0.07. The two subsequent flares were consistent with the reheating of the same magnetic structure as of the first flare. The large-scale magnetic structure and the periodicity of the flares imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation.

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arXiv1999

Rotation and X-ray emission from protostars

The ASCA satellite has recently detected variable hard X-ray emission from two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of quasi-periodic energetic flares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. In WL6, X-ray flaring is rotationally modulated, and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is a fast rotator (near break-up), while WL6 rotates with a significantly longer period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the central stars of YLW15 and WL6 respectively. On the long term, the interactions between the star and the disk results in magnetic braking and angular momentum loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same order as the estimated duration of the Class~I protostar stage. Close to the birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star, such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their lower-mass counterparts have had the time to spin down. The rapid rotation and strong star-disk magnetic interactions of YLW15 also naturally explain the observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the first clues to the rotation status and evolution of protostars.

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