The 2016-2017 peak luminosity of the pre-main sequence variable V2492 Cyg

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.Comment: Accepted by A&

When It Isn’t Always Lyme: Expanding the Differential Diagnosis for Acute-Onset Polyarthralgia in the West Virginia Eastern Panhandle

This case presentation discusses a 36 year-old female animal care worker presenting with an acute-onset polyarthropathy during the summer months in a Lyme endemic region. Though she appeared to be a good candidate for the diagnosis of Lyme borreliosis, her screening serology reported negative results and alternative diagnoses were considered. Her subsequent diagnosis with parvovirus B19 acts to remind the general practitioner to have confidence in the accuracy of a negative Lyme screen and, upon negative result, to expand the differential to include less common infections including parvovirus B19. It also highlights the need to remember parvovirus B19 in a similar patient who has the potential to be or to become pregnant during the course of her illness

Simultaneous monitoring of the photometric and polarimetric activity of the young star PV Cep in the optical/near-infrared bands

We present the results of a simultaneous monitoring, lasting more than 2 years, of the optical and near-infrared photometric and polarimetric activity of the variable protostar PV Cep. During the monitoring period, an outburst has occurred in all the photometric bands, whose declining phase ($\Delta$J $\approx$ 3 mag) lasted about 120 days. A time lag of $\sim$ 30 days between optical and infrared light curves has been measured and interpreted in the framework of an accretion event. This latter is directly recognizable in the significant variations of the near-infrared colors, that appear bluer in the outburst phase, when the star dominates the emission, and redder in declining phase, when the disk emission prevails. All the observational data have been combined to derive a coherent picture of the complex morphology of the whole PV Cep system, that, in addition to the star and the accretion disk, is composed also by a variable biconical nebula. In particular, the mutual interaction between all these components is the cause of the high value of the polarization ($\approx$ 20%) and of its fluctuations. The observational data concur to indicate that PV Cep is not a genuine EXor star, but rather a more complex object; moreover the case of PV Cep leads to argue about the classification of other recently discovered young sources in outburst, that have been considered, maybe over-simplifying, as EXor.Comment: Accepted for publication on Ap

Pre-Main Sequence variables in the VMR-D : identification of T Tauri-like accreting protostars through Spitzer-IRAC variability

We present a study of the infrared variability of young stellar objects by means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained in observations separated in time by about six months. By using the same space-born IR instrumentation, this study eliminates all the unwanted effects usually unavoidable when comparing catalogs obtained from different instruments. The VMR-D map covers about 1.5 square deg. of a site where star formation is actively ongoing. We are interested in accreting pre-main sequence variables whose luminosity variations are due to intermittent events of disk accretion (i.e. active T Tauri stars and EXor type objects). The variable objects have been selected from a catalog of more than 170,000 sources detected at a S/N ratio > 5. We searched the sample of variables for ones whose photometric properties are close to those of known EXor's. These latter are monitored in a more systematic way than T Tauri stars and the mechanisms that regulate the observed phenomenology are exactly the same. Hence the modalities of the EXor behavior is adopted as driving criterium for selecting variables in general. We selected 19 bona fide candidates that constitute a well-defined sample of new variable targets for further investigation. Out of these, 10 sources present a Spitzer MIPS 24 micron counterpart, and have been classified as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources have spectral energy distribution compatible with phases older than Class I. This is consistent with what is known about the small sample of known EXor's, and suggests that the accretion flaring or EXor stage might come as a Class I/II transition. We present also new prescriptions that can be useful in future searches for accretion variables in large IR databases.Comment: 35 pages, 12 figures To appear in Ap

On the 2015 outburst of the EXor variable star V1118 Ori

After a long-lasting period of quiescence of about a decade, the source V1118 Ori, one of the most representative members of the EXor variables, is now outbursting. Since the initial increase of the near-infrared flux of about 1 mag (JHK bands) registered on 2015 September 22, the source brightness has remained fairly stable. We estimate DeltaV about 3 mag with respect to the quiescence phase. An optical/near-IR low-resolution spectrum has been obtained with the Large Binocular Telescope instruments MODS and LUCI2, and compared with a spectrum of similar spectral resolution and sensitivity level taken during quiescence. Together with the enhancement of the continuum, the outburst spectrum presents a definitely higher number of emission lines, in particular HI recombination lines of the Balmer, Paschen, and Brackett series, along with bright permitted lines of several species, forbidden atomic lines, and CO ro-vibrational lines. Both mass accretion and mass loss rates have significantly increased (by to about an order of magnitude, mass accretion rate = 1.2-4.8 10^-8 M_sun/yr, mass loss rate = 0.8-2 10^-9 M_sun/yr) with respect to the quiescence phase. If compared with previous outbursts, the present one appears less energetic. Alternatively, it could already be in the fading phase (with the maximum brightness level reached when the source was not visible), or, viceversa, still in the rising phase.Comment: Accepted ApJ Letter

POISSON project - III - Investigating the evolution of the mass accretion rate

As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT), we present the results of the analysis of low-resolution NIR spectra 0.9-2.4 um) of two samples of YSOs in Lupus and Serpens (52 and 17 objects), with masses 0.1-2.0 Msun and ages from 10^5 to a few 10^7 yr. After determining the accretion parameters of the Lup and Ser targets by analysing their HI near-IR emission features, we added the results to those from previous regions (investigated in POISSON with the same methodology). We obtained a final catalogue (143 objects) of mass accretion rates (Macc) derived in a homogeneous fashion and analysed how Macc correlates with M* and how it evolves in time. We derived the accretion luminosity (Lacc) and Macc for Lup and Ser objects from the Br_gamma line by using relevant empirical relationships from the literature that connect HI line luminosity and Lacc. To minimise the biases and also for self-consistency, we re-derived mass and age for each source using the same set of evolutionary tracks. We observe a correlation MaccM*^2.2, similarly to what has previously been observed in several star-forming clouds. The time variation of Macc is roughly consistent with the expected evolution in viscous disks, with an asymptotic decay that behaves as t^-1.6. However, Macc values are characterised by a large scatter at similar ages and are on average higher than the predictions of viscous models. Although part of the scattering may be related to the employed empirical relationship and to uncertainties on the single measurements, the general distribution and decay trend of the Macc points are real. These findings might be indicative of a large variation in the initial mass of the disks, of fairly different viscous laws among disks, of varying accretion regimes, and of other mechanisms that add to the dissipation of the disks, such as photo-evaporation.Comment: 18 pages, 10 figures, accepted by A&

On the nature of the EXor accretion events: an unfrequent manifestation of a common phenomenology ?

We present the results of a comparison between classical and newly identified EXor based on literature data and aimed at recognizing possible differences or similarities of both categories. Optical and near-IR two-color diagrams, modalities of fluctuations, and derived values of the mass accretion rates are indicative of strong similarities between the two samples. We demonstrate how the difference between the outburst and the quiescence spectral energy distribution of all the EXor can be well fitted with a single blackbody, as if an additional thermal component appears during the outbursting phase. Temperatures of this additional component span between 1000 and 4500 K, while the radii of the emitting regions (assumed to be a uniform disk) span between 0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk. Spots persisting up to 50% of the outburst duration, not exceeding the 10% of the stellar surface, and with temperatures compatible with the EXor mass accretion rates, are able to account for both the appearance of the additional thermal component and the dust sublimation in the inner structures of the disk. We also compare the EXor events with the most significant color and magnitude fluctuations of active T Tauri stars finding that (i} burst accretion phenomena should also be important for this latter class; (ii} EXor events could be more frequent then those accidentally discovered. Remarkable is the case of the source V2493 Cyg, a T Tauri star recently identified as a strong outbursting object: new optical and near-IR photometric and spectroscopic data are presented trying to clarify its EXor or FUor nature.Comment: Accepted for publication in Ap