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Register a new userThe 2016-2017 peak luminosity of the pre-main sequence variable V2492 Cyg
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V2492 Cyg is a young pre-main sequence star presenting repetitive brightness variations of significant amplitude (Delta R > 5 mag) whose physical origin has been ascribed to both extinction (UXor-type) and accretion (EXor-type) variability, although their mutual proportion has not been clarified yet. Recently, V2492 Cyg has reached a level of brightness ever registered in the period of its documented activity. Optical and near-infrared photometry and spectroscopy have been obtained in October 2016 and between March and July 2017. The source has remained bright until the end of May 2017, then it started to rapidly fade since the beginning of June at a rate of about 0.08 mag/day. On mid-July 2017 the source has reached the same low-brightness level as two years before. Extinction and mass accretion rate were derived by means of the luminosity of the brightest lines, in particular Halpha and Hbeta. A couple of optical high-resolution spectra are also presented to derive information on the gas kinematics. Visual extinction variations do not exceed a few magnitudes, while the mass accretion rate is estimated to vary from less than 10^-8 up to a few 10^-7 M_sun/yr. This latter is comparable to that estimated on the previous high-state in 2010, likely occurred under more severe extinction conditions. The combined analysis of the optical and near-infrared (NIR) observations extends to the present event the original suggestion that the V2492 Cyg variability is a combination of changing extinction and accretion.
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The bulk of X-ray emission from pre-main-sequence (PMS) stars is coronal in origin. We demonstrate herein that stars on Henyey tracks in the Hertzsprung-Russell diagram have lower $log(L_X/L_ast)$, on average, than stars on Hayashi tracks. This effect is driven by the decay of $L_X$ once stars develop radiative cores. $L_X$ decays faster with age for intermediate mass PMS stars, the progenitors of main sequence A-type stars, compared to those of lower mass. As almost all main sequence A-type stars show no detectable X-ray emission, we may already be observing the loss of their coronae during their PMS evolution. Although there is no direct link between the size or mass of the radiative core and $L_X$, the longer stars have spent with partially convective interiors, the weaker their X-ray emission becomes. This conference paper is a synopsis of Gregory, Adams and Davies (2016).
We present results of multi-epoch (fourteen nights during 2007-2010) $V$-band photometry of the cluster NGC 1893 region to identify photometric variable stars in the cluster. The study identified a total of 53 stars showing photometric variability. The members associated with the region are identified on the basis of spectral energy distribution, $J-H/H-K$ two colour diagram and $V/V-I$ colour-magnitude diagram. The ages and masses of the majority of pre-main-sequence sources are found to be $lesssim$ 5 Myr and in the range 0.5 $lesssim$ $M/M_{odot}$ $lesssim$ 4, respectively. These pre-main-sequence sources hence could be T Tauri stars. We also determined the physical parameters like disk mass and accretion rate from the spectral energy distribution of these T Tauri stars. The periods of majority of the T Tauri stars range from 0.1 to 20 day. The brightness of Classical T Tauri stars is found to vary with larger amplitude in comparison to Weak line T Tauri stars. It is found that the amplitude decreases with increase in mass, which could be due to the dispersal of disks of massive stars.
We have investigated the nature of the variability of CHS7797, an unusual periodic variable in the Orion Nebula Cluster. An extensive I-band photometric data set of CHS7797 was compiled between 2004-2010 using various telescopes. Further optical data have been collected in R and z bands. In addition, simultaneous observations of the ONC region including CHS7797 were performed in the I, J, Ks and IRAC [3.6] and [4.5] bands over a time interval of about 40d. CHS7797 shows an unusual large-amplitude variation of about 1.7 mag in the R, I, and z bands with a period 17.786. The amplitude of the brightness modulation decreases only slightly at longer wavelengths. The star is faint during 2/3 of the period and the shape of the phased light-curves for seven different observing seasons shows minor changes and small-amplitude variations. Interestingly, there are no significant colour-flux correlations for wavelengths smaller than 2microns, while the object becomes redder when fainter at longer wavelengths. CHS7797 has a spectral type of M6 and an estimated mass between 0.04-0.1Msun. The analysis of the data suggests that the periodic variability of CHS7797 is most probably caused by an orbital motion. Variability as a result of rotational brightness modulation by spots is excluded by the lack of any color-brightness correlation in the optical. The latter indicates that CHS7797 is most probably occulted by circumstellar matter in which grains have grown from typical 0.1 microns to 1-2 micron sizes. We discuss two possible scenarios in which CHS7797 is periodically eclipsed by structures in a disc, namely that CHS7797 is a single object with a circumstellar disc, or that CHS7797 is a binary system, similar to KH15D, in which an inclined circumbinary disc is responsible of the variability. Possible reasons for the typical 0.3mag variations in I-band at a given phase are discussed.
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 mu m and 2.2 mu m from the ground and between 3.6 mu m and 160 mu m from space. Methods. We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results. We find that the shapes of the light curves at different wavelengths are strictly self-similar and that the observed variability is related to a single physical process, most likely variable extinction. We suggest that the central source is episodically occulted by a dense dust cloud in the inner disk, and, based on the invariability of the far-infrared fluxes, we propose that it is a long-lived rather than a transient structure. In some respects, V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars. Conclusions. The example of V2492 Cyg demonstrates that the light variations of young eruptive stars are not exclusively related to changing accretion. The variability provided information on an azimuthally asymmetric structural element in the inner disk. Such an asymmetric density distribution in the terrestrial zone may also have consequences for the initial conditions of planet formation.
Correlations between the accretion luminosity and emission line luminosities (L_acc and L_line) of pre-main sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) L_acc-L_line correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L_star) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the L_acc-L_line correlations depend on the L_acc-L_star relationship. We conclude that because PMS stars show the L_acc-L_star correlation immediately implies that L_acc also correlates with the luminosity of all emission lines, for which the L_acc-L_line correlations alone do not prove any physical connection with accretion but can only be used with practical purposes to roughly estimate accretion rates. When looking for correlations with possible physical meaning, we suggest that L_acc/L_star and L_line/L_star should be used instead of L_acc and L_line. Finally, the finding that L_acc has a steeper dependence on L_star for T-Tauri stars than for intermediate-mass Herbig Ae/Be stars is also discussed. That is explained from the magnetospheric accretion scenario and the different photospheric properties in the near-UV.
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