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Register a new userMACHO 311.37557.169: A VY Scl star
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Optical surveys, such as the MACHO project, often uncover variable stars whose classification requires followup observations by other instruments. We performed X-ray spectroscopy and photometry of the unusual variable star MACHO 311.37557.169 with XMM in April 2018, supplemented by archival X-ray and optical spectrographic data. The star has a bolometric X-ray luminosity of about $1times 10^{32}$ erg s$^{-1}$ cm$^{-2}$ and a heavily absorbed two-temperature plasma spectrum. The shape of its light curve, its overall brightness, its X-ray spectrum, and the emission lines in its optical spectrum suggest that it is most likely a VY~Scl cataclysmic variable.
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Four VY Scl-type nova-like systems were observed in X-rays during both the low and the high optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari, and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multi-component model of plasma in collisional ionization equilibrium. The high state high resolution spectra of TT Ari taken with Chandra ACIS-S and the HETG gratings show a rich emission line spectrum, with prominent lines of in Mg, Si, Ne, and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disk emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.
We present the first time-resolved spectroscopic study of the cataclysmic variable DW Cancri. We have determined an orbital period of 86.10 +- 0.05 min, which places the system very close to the observed minimum period of hydrogen-rich cataclysmic variables. This invalidates previous speculations of DW Cnc being either a permanent superhumper below the period minimum or a nova-like variable with an orbital period longer than 3 hours showing quasi-periodic oscillations. The Balmer and HeI lines have double-peaked profiles and exhibit an intense S-wave component moving with the orbital period. Remarkably, the Balmer and HeI radial velocity curves are modulated at two periods: 86.10 +- 0.05 min (orbital) and 38.58 +- 0.02 min. The same short period is found in the equivalent width variations of the single-peaked HeII 4686 line. We also present time-resolved photometry of the system which shows a highly-coherent variation at 38.51 min, consistent with the short spectroscopic period. The large number of similarities with the short-period intermediate polar V1025 Cen lead us to suggest that DW Cnc is another intermediate polar below the period gap, and we tentatively identify the photometric and spectroscopic 38-min signals with the white dwarf spin period. DW Cnc has never been observed to undergo an outburst, but it occasionally exhibits low states ~2 mag fainter than its typical brightness level of V~14.5, resembling the behaviour of the high mass-transfer VY Scl stars.
We measure the broad emission line region (BLR) size of a luminous, L~1E47 erg/s, high-z quasar using broadband photometric reverberation mapping. To this end, we analyze ~7.5 years of photometric data for MACHO 13.6805.324 (z~1.72) in the B and R MACHO bands and find a time delay of 180+/-40 days in the rest frame of the object. Given the spectral-variability properties of high-z quasars, we associate this lag with the rest-UV iron emission blends. Our findings are consistent with a simple extrapolation of the BLR size-luminosity relation in local active galactic nuclei to the more luminous, high-z quasar population. Long-term spectroscopic monitoring of MACHO 13.6805.324 may be able to directly measure the line-to-continuum time-delay and test our findings.
Abridged. The 12CO/13CO ratio in the circumstellar envelope (CSE) of asymptotic giant branch (AGB) stars has been extensively used as the tracer of the photospheric 12C/13C ratio. However, spatially-resolved ALMA observations of R Scl, a carbon rich AGB star, have shown that the 12CO/13CO ratio is not consistent over the entire CSE. Hence, it can not necessarily be used as a tracer of the 12C/13C ratio. The most likely hypothesis to explain the observed discrepancy between the 12CO/13CO and 12C/13C ratios is CO isotopologue selective photodissociation by UV radiation. Unlike the CO isotopologue ratio, the HCN isotopologue ratio is not affected by UV radiation. Therefore, HCN isotopologue ratios can be used as the tracer of the atomic C ratio in UV irradiated regions. We have performed a detailed non-LTE excitation analysis of circumstellar H12CN and H13CN line emission around R Scl, observed with ALMA and APEX, using a radiative transfer code, ALI. The spatial extent of the molecular distribution for both isotopologues is constrained based on the spatially resolved H13CN(4-3) ALMA observations. We find fractional abundances of H12CN/H2 = (5.0 +- 2.0) x 10^{-5} and H13CN/H2 = (1.9 +- 0.4) x 10^{-6} in the inner wind (r < (2.0 +- 0.25) x 10^{15} cm) of R Scl. The derived circumstellar isotopologue ratio of H12CN/H13CN = 26.3 +- 11.9 is consistent with the photospheric ratio of 12C/13C ~ 19 pm 6. We show that the circumstellar H12CN/H13CN ratio traces the photospheric 12C/13C ratio. These results support the previously proposed explanation that CO isotopologue selective-shielding is the main factor responsible for the observed discrepancy between 12C/13C and 12CO/13CO ratios in the inner CSE of R Scl. This indicates that UV radiation impacts on the CO isotopologue ratio.
Recent observations of the dwarf elliptical galaxy Scl-dE1 (Sc22) in the Sculptor group of galaxies revealed an extended globular cluster (Scl-dE1 GC1), which exhibits an extremely large core radius of about 21.2 pc. The authors of the discovery paper speculated on whether this object could reside in its own dark matter halo and/or if it might have formed through the merging of two or more star clusters. In this paper, we present N-body simulations to explore thoroughly this particular formation scenario. We follow the merger of two star clusters within dark matter haloes of a range of masses (as well as in the absence of a dark matter halo). In order to obtain a remnant which resembles the observed extended star cluster, we find that the star formation efficiency has to be quite high (around 33 per cent) and the dark matter halo, if present at all, has to be of very low mass, i.e. raising the mass to light ratio of the object within the body of the stellar distribution by at most a factor of a few. We also find that expansion of a single star cluster following mass loss provides another viable formation path. Finally, we show that future measurements of the velocity dispersion of this system may be able to distinguish between the various scenarios we have explored.
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