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تسجيل مستخدم جديدMultiple protostellar systems. I. A deep near infrared survey of Taurus and Ophiuchus protostellar objects
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تأليف
G. Duchene
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(Abridged) We performed a deep infrared imaging survey of 63 embedded young stellar objects (YSOs) located in the Taurus and Ophiuchus clouds to search for companions. The sample includes Class I and flat infrared spectrum protostellar objects. We find 17 companions physically bound to 15 YSOs with angular separations in the range 0.8-10 (110-1400 AU) and derive a companion star fraction of 23+/-9 % and 29+/-7 % for embedded YSOs in Taurus and Ophiuchus, respectively. In spite of different properties of the clouds and especially of the prestellar cores, the fraction of wide companions, 27+/-6 % for the combined sample, is identical in the two star-forming regions. This suggests that the frequency and properties of wide multiple protostellar systems are not very sensitive to specific initial conditions. Comparing the companion star fraction of the youngest YSOs still surrounded by extended envelopes to that of more evolved YSOs, we find evidence for a possible evolution of the fraction of wide multiple systems, which seems to decrease by a factor of about 2 on a timescale of about 10^5 yr. Somewhat contrary to model predictions, we do not find evidence for a sub-clustering of embedded sources at this stage on a scale of a few 100 AU that could be related to the formation of small-N protostellar clusters. Possible interpretations for this discrepancy are discussed.
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(abridged) Our project endeavors to obtain a robust view of multiplicity among embedded Class I and Flat Spectrum protostars in a wide array of nearby molecular clouds to disentangle ``universal from cloud-dependent processes. We have used near-infra
red adaptive optics observations at the VLT through the H, Ks and L filters to search for tight companions to 45 Class I and Flat Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga, Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with published high-resolution surveys of 13 additional objects in Taurus and Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400 AU and 14-1400 AU separation ranges, respectively. These rates are in excellent agreement with those previously found among T Tauri stars in Taurus and Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate of solar-type field stars. We found no non-hierarchical triple systems, nor any quadruple or higher-order systems. No significant cloud-to-cloud difference has been found, except for the fact that all companions to low-mass Orion protostars are found within 100 AU of their primaries whereas companions found in other clouds span the whole range probed here. Based on this survey, we conclude that core fragmentation always yields a high initial multiplicity rate, even in giant molecular clouds such as the Orion cloud or in clustered stellar populations as in Serpens, in contrast with predictions of numerical simulations. The lower multiplicity rate observed in clustered Class II and Class III populations can be accounted for by a universal set of properties for young systems and subsequent ejections through close encounters with unrelated cluster members.
The GLIMPSE archive was used to obtain 3.6--8.0micron, point source photometry and images for 381 massive protostellar candidates lying in the Galactic mid-plane. The colours, magnitudes and spectral indicies of sources in each of the 381 target fiel
ds were analysed and compared with the predictions of 2D radiative transfer model simulations. Although no discernable embedded clusters were found in any targets, multiple sources or associations of redenned young stellar objects were found in many sources indicating multiplicity at birth. The spectral index ($alpha$) of these point sources in 3.6--8.0mum bands display large values of $alpha$=2--5. A color-magnitude analog plot was used to identify 79 infrared counterparts to the HMPOs. Compact nebulae are found in 75% of the detected sources with morphologies that can be well described by core-halo, cometary, shell-like and bipolar geometries similar to those observed in ultra-compact HII regions. The IRAC band SEDs of the IR counterparts of HMPOs are best described to represent YSOs with a mass range of 8--20msun in their Class I stages when compared with 2D radiative transfer models. They also suggest that the high $alpha$ values represent reprocessed star/star+disk emission that is arising in the dense envelopes. Thus we are witnessing the luminous envelopes around the protostars rather than their photospheres or disks. We argue that the compact infrared nebulae likely reflect the underlying physical structure of the dense cores and are found to imitate the morphologies of known UCHII regions. Our results favour models of continuuing accretion involving both molecular and ionised accretion components to build the most massive stars rather than purely molecular rapid accretion flows.
We present high resolution (R = 18,000), high signal-to-noise, 2 micron spectra of 52 infrared-selected Class I and flat-spectrum young stellar objects in the Taurus-Auriga, $rho$ Ophiuchi, Serpens, Perseus, and Corona Australis dark clouds. We detec
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cted which we designate Oph A-N1 through N6. The N4 and N5 maxima are coincident with the starless continuum objects SM1 and SM2 respectively but the other maxima are not coincident with previously-identified objects. In contrast, relatively little N2H+ 1-0 emission is coincident with the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the N2H+ 1-0 line, Delta V, varies by a factor of ~5 across Oph A. Values of Delta V < 0.3 km/s are found in 14 locations in Oph A, but only that associated with N6 is both well-defined spatially and larger than the beam size. Centroid velocities of the line, V_LSR, vary relatively little, having an rms of only ~0.17 km/s. Small-scale V_LSR gradients of <0.5 km/s over ~0.01 pc are found near SM1, SM1N, and SM2, but not N6. The low N2H+ abundances of SM2 or VLA 1623 relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2 adsorption onto dust grains in their colder and probably denser interiors. The low Delta V of N6, i.e., 0.193 km/s FWHM, is only marginally larger than the FWHM expected from thermal motions alone, suggesting turbulent motions in the Oph A core have been reduced dramatically at this location. The non-detection of N6 in previous thermal continuum maps suggests that interesting sites possibly related to star formation may be overlooked in such data.
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