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Hauyu Baobab Liu Mr.
Publication date
2013
fields
Physics
and research's language is
English
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We report JVLA 8-10 GHz ($lambda$=3.0-3.7 cm) monitoring observations toward the YSO cluster R Coronae Australis (R,CrA), taken in 2012, from March 15 to September 12. These observations were planned to measure the radio flux variabilities in timescales from 0.5 hours to several days, to tens of days, and up to $sim$200 days. We found that among the YSOs detectable in individual epochs, in general, the most reddened objects in the textit{Spitzer} observations show the highest mean 3.5 cm Stokes textit{I} emission, and the lowest fractional variabilities on $<$200-day timescales. The brightest radio flux emitters in our observations are the two reddest sources IRS7W and IRS7E. In addition, by comparing with observations taken in 1996-1998 and 2005, we found that the radio fluxes of these two sources have increased by a factor $sim$1.5. The mean 3.5-cm fluxes of the three Class I/II sources IRSI, IRS2, and IRS6 appear to be correlated with their accretion rates derived by a previous near infrared line survey. The weakly accreting Class I/II YSOs, or those in later evolutionary stages, present radio flux variability on $<$0.5-hour timescales. Some YSOs were detected only during occasional flaring events. The source R,CrA went below our detection limit during a few fading events.
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Jets and outflows are ubiquitous in the process of formation of stars since outflow is intimately associated with accretion. Free-free (thermal) radio continuum emission is associated with these jets. This emission is relatively weak and compact, and sensitive radio interferometers are required to study it. Observations in the cm range are most useful to trace the base of the ionized jets, close to the central protostar, where optical or near-IR imaging is made difficult by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale. Future instruments such as the Square Kilometre Array (SKA) and the Next Generation Very Large Array (ngVLA) will be crucial to perform this kind of sensitive observations. Thermal jets are associated with both high and low mass protostars and possibly even with substellar objects. The ionizing mechanism of these radio jets appears to be related to shocks in the associated outflows, as suggested by the observed correlation between the cm luminosity and the outflow momentum rate. Some protostellar jets show indications of non-thermal emission in their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80-81, allowing one to map the jet magnetic field, a key ingredient to determine the collimation and ejection mechanisms. As only a fraction of the emission is polarized, very sensitive observations such as those that will be feasible with the interferometers previously mentioned are required to perform studies in a large sample of sources. Jets are present in many kinds of astrophysical scenarios. Characterizing radio jets in young stars, where thermal emission allows one to determine their physical conditions, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets.
We present 450 and 850 um maps of R Coronae Australis. We compare the maps to previous surveys of the region, and shed new light on the previously unknown nature of the protostellar sources at the centre of the cloud. We clarify the nature of two millimetre sources previously discovered in lower resolution data. We identify one new Class 0 protostar that we label SMM 1B, and we measure the envelope masses of a number of more evolved protostars. We identify two new prestellar cores that we call SMM 1A and SMM 6.
The R CrA region was observed in the 3.5 and 6.2 cm continuum with high angular resolutions (0.6--1.7 arcseconds) using the Very Large Array. Archival data sets were also analyzed for comparison, which provided angular resolutions up to 0.3 arcseconds. A cluster of young stellar objects was detected, and a rich array of star forming activities was revealed. IRS 7A showed an enhanced outflow activity recently. The main peak of IRS 7A positionally coincides with an X-ray source, which suggests that the X-ray emission is directly related to the central protostar. The Class 0 source SMA 2 is associated with a double radio source, B 9a and 9b, and seems to be driving two outflows. The B 9 complex is probably a multiple-protostar system. Both B 9a and 9b are nonthermal radio sources with negative spectral indices. IRS 7B is a compact radio source surrounded by an extended structure. The compact source corresponds to the Class 0/I source SMA 1, and it is also closely associated with an X-ray source, suggesting that magnetic activities start early in the protostellar stage of evolution. The extended structure of IRS 7B may be a bipolar outflow. IRS 5 was resolved into two sources with a separation of 0.9 arcseconds. Both IRS 5a and 5b display radio flares and X-ray emission, suggesting that energetic magnetic processes are active in both members. The month-scale active phase of IRS 5b implies that the flare activity must involve large-scale magnetic fields. During the strong flare event of IRS 5b in 1998, IRS 5a also showed an enhanced level of radio emission. This concurrent activity suggests that IRS 5 may be an interacting young binary system, but the interaction mechanism is unknown. Alternatively, what was seen in the radio images could be a circumbinary halo.
Circularly polarized 3.5 cm continuum emission was detected toward three radio sources in the R CrA region using the Very Large Array. The Class I protostar IRS 5b persistently showed polarized radio emission with a constant helicity over 8 yr, which suggests that its magnetosphere has a stable configuration. There is a good correlation between the Stokes I and Stokes V fluxes, and the fractional polarization is about 0.17. During active phases the fractional polarization is a weakly decreasing function of Stokes I flux, which suggests that IRS 5b is phenomenologically similar to other types of flare stars such as RS CVn binaries. The variability timescale of the polarized flux is about a month, and the magnetosphere of IRS 5b must be very large in size. The Class I protostar IRS 7A was detected once in circularly polarized radio emission, even though IRS 7A drives a thermal radio jet. This detection implies that the radio emission from the magnetosphere of a young protostar can escape the absorption by the partially ionized wind at least once in a while. The properties of IRS 7A and IRS 5b suggests that Class I protostars have organized peristellar magnetic fields of a few kilogauss and that the detectability of magnetospheric emission may depend on the evolutionary status of protostar. Also reported is the detection of circularly polarized radio emission toward the variable radio source B5.
Optical high-resolution spectra of the R Coronae Borealis star V CrA at light maximum and during minimum light arediscussed. Abundance analysis confirms previous results showing that V CrA has the composition of the small subclass of R Coronae Borealis (RCB) stars know as `minority RCBs, i.e., the Si/Fe and S/Fe ratios are 100 times their solar values. A notable novel result for RCBs is the detection of the 1-0 Swan system $^{12}$C$^{13}$C bandhead indicating that $^{13}$C is abundant: spectrum synthesis shows that $^{12}$C/$^{13}$C is about 3 to 4. Absorption line profiles are variable at maximum light with some lines showing evidence of splitting by about 10 km s$^{-1}$. A spectrum obtained as the star was recovering from a deep minimum shows the presence of cool C$_2$ molecules with a rotational temperature of about 1200K, a temperature suggestive of gas in which carbon is condensing into soot. The presence of rapidly outflowing gas is shown by blue-shifted absorption components of the Na {sc i} D and K {sc i} 7698 AA resonance lines.
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