Towards a better classification of unclear eruptive variables: the cases of V2492 Cyg, V350 Cep, and ASASSN-15qi

Eruptive variables are young stars that show episodic variations of brightness: EXors/FUors variations are commonly associated with enhanced accretion outbursts occurring at intermittent cadence of months/years (EXors) and decades/centuries (FUors). Variations that can be ascribed to a variable extinction along their line of sight are instead classified as UXors. We aim at investigating the long-term photometric behaviour of three sources classified as eruptive variables. We present data from the archival plates of the Asiago Observatory relative to the fields where the targets are located. For the sake of completeness we have also analysed the Harvard plates of the same regions that cover a much longer historical period, albeit at a lower sensitivity, however we are only able to provide upper limits. A total of 273 Asiago plates were investigated, providing a total of more than 200 magnitudes for the three stars, which cover a period of about 34 yr between 1958 and 1991. We have compared our data with more recently collected literature data. Our plates analysis of V2492 Cyg provides historical upper limits that seem not to be compatible with the level of the activity monitored during the last decade. Therefore, recently observed accretion phenomena could be associated with the outbursting episodes, more than repetitive obscuration. While a pure extinction does not seem the only mechanism responsible for the ASASSN-15qi fluctuations, it can account quite reasonably for the recent V350 Cep variations.Comment: 12 pages, accepted by A&

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

The HH34 outflow as seen in [FeII]1.64um by LBT-LUCI

Dense atomic jets from young stars copiously emit in [FeII] IR lines, which can, therefore, be used to trace the immediate environments of embedded protostars. We want to investigate the morphology of the bright [FeII] 1.64um line in the jet of the source HH34 IRS and compare it with the most commonly used optical tracer [SII]. We analyse a 1.64um narrow-band filter image obtained with the Large Binocular Telescope (LBT) LUCI instrument, which covers the HH34 jet and counterjet. A Point Spread Function (PSF) deconvolution algorithm was applied to enhance spatial resolution and make the IR image directly comparable to a [SII] HST image of the same source. The [FeII] emission is detected from both the jet, the (weak) counter-jet, and from the HH34-S and HH34-N bow shocks. The deconvolved image allows us to resolve jet knots close to about 1\arcsec from the central source. The morphology of the [FeII] emission is remarkably similar to that of the [SII] emission, and the relative positions of [FeII] and [SII] peaks are shifted according to proper motion measurements, which were previously derived from HST images. An analysis of the [FeII]/[SII] emission ratio shows that Fe gas abundance is much lower than the solar value with up to 90% of Fe depletion in the inner jet knots. This confirms previous findings on dusty jets, where shocks are not efficient enough to remove refractory species from grains.Comment: 5 pages, 4 figures, note accepted by A&

Spitzer-MIPS survey of the young stellar content in the Vela Molecular Cloud-D

A new, unbiased Spitzer-MIPS imaging survey (~1.8 square degs) of the young stellar content of the Vela Molecular Cloud-D is presented. The survey is complete down to 5mJy and 250mJy at 24micron (mu) and 70mu, respectively. 849 sources are detected at 24mu and 52 of them also have a 70mu counterpart. The VMR-D region is one that we have already partially mapped in dust and gas millimeter emission, and we discuss the correlation between the Spitzer compact sources and the mm contours. About half of the 24mu sources are located inside the region delimited by the 12CO(1-0) contours (corresponding to only one third of the full area mapped with MIPS) with a consequent density increase of about 100% of the 24mu sources [four times for 70mu ones] moving from outside to inside the CO contours. About 400 sources have a 2MASS counterpart. So we have constructed a Ks vs. Ks-[24] diagram and identified the protostellar population. We find an excess of Class I sources in VMR-D in comparison with other star forming regions. This result is reasonably biased by the sensitivity limits, or, alternatively, may reflect a very short lifetime (<=10^6yr) of the protostellar content in this cloud. The MIPS images have identified embedded cool objects in most of the previously identified starless cores; in addition, there are 6 very young, possibly Class 0 objects identified. Finally we report finding of the driving sources for a set of five out of six very compact protostellar jets previously discovered in near-infrared images.Comment: 29 pages, 14 figures. To appear in Ap.

On the binarity of the classical Cepheid X Sgr from interferometric observations

Optical-infrared interferometry can provide direct geometrical measurements of the radii of Cepheids and/or reveal unknown binary companions of these stars. Such information is of great importance for a proper calibration of Period-Luminosity relations and for determining binary fraction among Cepheids. We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprove the presence of the hypothesized binary companion and to directly measure the mean stellar radius, possibly detecting its variation along the pulsation cycle. From AMBER observations in MR mode we performed a binary model fitting on the closure phase and a limb-darkened model fitting on the visibility. Our analysis indicates the presence of a point-like companion at a separation of 10.7 mas and 5.6 magK fainter than the primary, whose flux and position are sharply constrained by the data. The radius pulsation is not detected, whereas the average limb-darkened diameter results to be 1.48+/-0.08 mas, corresponding to 53+/-3 R_sun at a distance of 333.3 pc.Comment: 5 pages, 3 figures, research not

Near- and Far-Infrared Counterparts of Millimeter Dust Cores in the Vela Molecular Ridge Cloud D

The aim of this paper is to identify the young protostellar counterparts associated to dust millimeter cores of the Vela Molecular Ridge Cloud D through new IR observations (H_2 narrow-band at 2.12 micron and N broad band at 10.4 micron) along with an investigation performed on the existing IR catalogues. The association of mm continuum emission with infrared sources from catalogues (IRAS, MSX, 2MASS), JHK data from the literature and new observations, has been established according to spatial coincidence, infrared colours and spectral energy distributions. Only 7 out of 29 resolved mm cores (and 16 out of the 26 unresolved ones) do not exhibit signposts of star formation activity. The other ones are clearly associated with: far-IR sources, H_2 jets or near-IR objects showing a high intrinsic colour excess. The distribution of the spectral indices pertaining to the associated sources is peaked at values typical of Class I objects, while three objects are signalled as candidates Class 0 sources. We remark the high detection rate (30%) of H_2 jets driven by sources located inside the mm-cores. They appear not driven by the most luminous objects in the field, but rather by less luminous objects in young clusters, testifying the co-existence of both low- and intermediate-mass star formation. The presented results reliably describe the young population of VMR-D. However, the statistical evaluation of activity vs inactivity of the investigated cores, even in good agreement with results found for other star forming regions, seems to reflect the limiting sensitivity of the available facilities rather than any property intrinsic to the mm-condensations.Comment: 38 pages. To be published to Astronomy & Astrophysic

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&