M. Kun

The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex

Context. In August 2010, the sudden optical brightening of two young stellar objects, HBC 722 and VSX J205126.1+440523, located in the North America/Pelican Nebula Complex, was announced. Early photometric and spectroscopic observations of these objects indicated that they may belong to the FUor or EXor class of young eruptive stars. The eruptions of FUors and EXors are often ex- plained by enhanced accretion of material from the circumstellar disk to the protostar. Aims. In order to determine the true nature of these two objects, we started an optical and near-infrared monitoring program, a nd complemented our data with archival observations and data from the literature. Methods. We plot and analyze pre-outburst and outburst spectral energy distributions (SEDs), multi-filter light curves, and col or-color diagrams. Results. The quiescent SED of HBC 722 is consistent with that of a slightly reddened normal T Tauri-type star. The source bright- ened monotonically in about two months, and the SED obtained during maximum brightness indicates the appearance of a hot, single-temperature blackbody. The current fading rate implies that the star will return to quiescence in about a year, q uestioning its classification as a bone fide FUor. The quiescent SED of VSX J20 5126.1+440523 looks like that of a highly embedded Class I source. The outburst of this source happened more gradually, but reached an unprecedentedly high amplitude. At 2.5 months after the peak, its light curves show a deep minimum, when the object was close to its pre-outburst optical brightness. Further monitori ng indicates that it is still far from being quiescent. Conclusions. The shape of the light curves, as well as the bolometric luminosities and accretion rates suggest that these objects do no t fit into the classic FUor group. Although HBC 722 exhibit all s pectral characteristics of a bona fide FUor, its luminosity a nd accretion rate is too low, and its timescale is too fast compared to clas sical FUors. VSX J205126.1+440523 seems to be an example where quick extinction changes modulate the light curve.

The young, massive, star cluster Sandage-96 after the explosion of supernova 2004dj in NGC 2403

The bright Type II-plateau supernova (SN) 2004dj occurred within the young, massive stellar cluster Sandage-96 in a spiral arm of NGC 2403. New multiwavelength observations obtained with several ground-based and space-based telescopes were combined to study the radiation from Sandage-96 after SN 2004dj faded away. Sandage-96 started to dominate the flux in the optical bands starting from 2006 September (~800 days after explosion). The optical fluxes are equal to the pre-explosion ones within the observational uncertainties. An optical Keck spectrum obtained ~900 days after explosion shows the dominant blue continuum from the cluster stars shortward of 6000? ? as well as strong SN nebular emission lines redward. The integrated spectral energy distribution (SED) of the cluster has been extended into the ultraviolet region by archival XMM-Newton and new Swift observations, and compared with theoretical models. The outer parts of the cluster have been resolved by the Hubble Space Telescope, allowing the construction of a color-magnitude diagram (CMD). The fitting of the cluster SED with theoretical isochrones results in cluster ages distributed between 10 and 40 Myr, depending on the assumed metallicity and the theoretical model family. The isochrone fitting of the CMDs indicates that the resolved part of the cluster consists of stars having a bimodal age distribution: a younger population at ~10-16 Myr and an older one at ~32-100? Myr. The older population has an age distribution similar to that of the other nearby field stars. This may be explained with the hypothesis that the outskirts of Sandage-96 are contaminated by stars captured from the field during cluster formation. The young age of Sandage-96 and the comparison of its pre and postexplosion SEDs suggest 12 M prog 20 M ? as the most probable mass range for the progenitor of SN 2004dj. This is consistent with, but perhaps slightly higher than, most of the other Type II-plateau SN progenitor masses determined so far.

The infrared properties of the new outburst star IRAS 05436-0007 in quiescent phase

We compiled and investigated the infrared/sub-mm/mm SED of the new outburst star IRAS 05436−0007 in quiescent phase. The star is a flat-spectrum source, with an estimated total luminosity of Lbol ≈ 5.6 L� , typical of low-mass T Tauri stars. The derived circumstellar mass of 0.5 Mis rather high among low-mass YSOs. The observed SED differs from the SEDs of typical T Tauri stars and of 4 well-known EXors, and resembles more the SEDs of FU Orionis objects indicating the presence of a circumstellar envelope. IRAS 05436−0007 seems to be a class II source with an age of approximately 4 × 10 5 yr. In this evolutionary stage an accretion disk is already fully developed, though a circumstellar envelope may also be present. Observations of the present outburst will provide additional knowledge on the source.

Lynds 1622: a nearby star-forming cloud projected on Orion B?

ABSTRACT We present results of optical spectroscopic and photometric observation of the pre-main sequence stars associated with the cometary shaped dark cloud Lynds 1622, and 12 CO and 13 CO observations of the cloud. We determined the effective temperaturesand luminosities of 14 pre-main sequence stars associated with the cloud from theirpositions in the Hertzsprung–Russell diagram, as well as constructed their spectralenergy distributions using optical, 2MASS and Spitzer IRAC and MIPS data. Wederived physical parameters of L1622 from the molecular observations. Our resultsare not compatible with the assumption that L1622 lies on the near side of the Orion–Eridanus loop, but suggest that L1622 is as distant as Orion B. At a distance of 400 pcthe mass of the cloud, derived from our 12 CO data, is 1100M⊙, its star formationefficiency is ∼ 1.8%, and the average age of its low-mass pre-main sequence starpopulation is about 1 million years.Key words: ISM: clouds; ISM: individual objects: L1622; stars: formation; stars:pre-main sequence

An all-sky support vector machine selection of WISE YSO candidates

We explored the AllWISE catalogue of the Wide-field Infrared Survey Explorer mission and identified Young Stellar Object candidates. Reliable 2MASS and WISE photometric data combined with Planck dust opacity values were used to build our dataset and to find the best classification scheme. A sophisticated statistical method, the Support Vector Machine (SVM) is used to analyse the multi-dimensional data space and to remove source types identified as contaminants (extragalactic sources, main sequence stars, evolved stars and sources related to the interstellar medium). Objects listed in the SIMBAD database are used to identify the already known sources and to train our method. A new all-sky selection of 133,980 Class I/II YSO candidates is presented. The estimated contamination was found to be well below 1% based on comparison with our SIMBAD training set. We also compare our results to that of existing methods and catalogues. The SVM selection process successfully identified >90% of the Class I/II YSOs based on comparison with photometric and spectroscopic YSO catalogues. Our conclusion is that by using the SVM, our classification is able to identify more known YSOs of the training sample than other methods based on colour-colour and magnitude-colour selection. The distribution of the YSO candidates well correlates with that of the Planck Galactic Cold Clumps in the Taurus–Auriga–Perseus–California region.

New homogeneous iron abundances of double-mode Cepheids from high-resolution echelle spectroscopy

Aims. We define the relationship between the double-mode pulsation of Cepheids and metallicity in a more accurate way, determine the empirical metallicities of double-mode Cepheids from homogeneous, high-resolution spectroscopic data, and study of the periodratio – metallicity dependence. Methods. The high S /N echelle spectra obtained with the FEROS spectrograph were analyzed using a self-developed IRAF script, and the iron abundances were determined by comparing with synthetic spectra assuming LTE. Results. Accurate [Fe/H] values of 17 galactic beat Cepheids were determined. All these stars have solar or slightly subsolar metallicity. Their period ratio (P1/P0) shows strong correlation with their derived [Fe/H] values. The corresponding period ratio – metallicity relation has been evaluated.

Mid-Infrared Spectral Variability Atlas of Young Stellar Objects

Optical and near-infrared variability is a well-known property of young stellar objects. However, a growing number of recent studies claim that a considerable fraction of them also exhibit mid-infrared flux changes. With the aim of studying and interpreting variability on a decadal timescale, here we present a mid-infrared spectral atlas containing observations of 68 low- and intermediate-mass young stellar objects. The atlas consists of 2.5-11.6 μm low-resolution spectra obtained with the ISOPHOT-S instrument on board the Infrared Space Observatory (ISO) between 1996 and 1998, as well as 5.2-14.5 μm low-resolution spectra obtained with the Infrared Spectrograph instrument on board the Spitzer Space Telescope between 2004 and 2007. The observations were retrieved from the ISO and Spitzer archives and were post-processed interactively by our own routines. For those 47 objects where multi-epoch spectra were available, we analyze mid-infrared spectral variability on annual and/or decadal timescales. We identify 37 variable candidate sources. Many stars show wavelength-independent flux changes, possibly due to variable accretion rates. In several systems, all exhibiting 10 μm silicate emission, the variability of the 6-8 μm continuum, and the silicate feature exhibit different amplitudes. A possible explanation is variable shadowing of the silicate-emitting region by an inner disk structure of changing height or extra silicate emission from dust clouds in the disk atmosphere. Our results suggest that mid-infrared variability, in particular, the wavelength-dependent changes, is more ubiquitous than was known before. Interpreting this variability is a new possibility for exploring the structure of the disk and its dynamical processes.

Exploring the circumstellar environment of the young eruptive star V2492 Cygni

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 μm and 2.2 μm from the ground and between 3.6 μm and 160 μm 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.

A peculiar young eruptive star in the dark cloud Lynds 1340

We conducted a long-term optical photometric and spectroscopic monitoring of the strongly variable, accreting young sun-like star [KOS94] HA11, associated with the dark cloud Lynds 1340 that exhibited large amplitude (5-6 mag in the I{sub C} band) brightness variations on 2-3 years timescales, flat spectral energy distribution (SED), and extremely strong (300{approx}< EW/A {approx}< 900) H{alpha} emission. In this Letter we describe the basic properties of the star, derived from our observations between 1999 and 2011, and put into context the observed phenomena. The observed variations in the emission spectra, near-infrared colors, and SED suggest that [KOS94] HA11 (spectral type: K7-M0) is an eruptive young star, possibly similar in nature to V1647 Ori: its large-scale photometric variations are governed by variable accretion rate, associated with variations in the inner disk structure. The star recently has undergone strong and rapid brightness variations, thus its further observations may offer a rare opportunity for studying structural and chemical rearrangements of the inner disk, induced by variable central luminosity.

Star count analysis of the interstellar matter in the region of L1251

We studied the ISM distribution in and around the star forming cloud L1251 with optical star counts. A careful calculation with a maximum likelihood based statistical approach resulted in B, V, R, I extinction distributions from the star count maps. A distance of 330 ± 30 pc was derived. The extinction maps revealed an elongated dense cloud with a bow shock at its eastern side. We estimated a Mach number of M ≈ 2 for the bow shock. A variation of the apparent dust properties is detected, i.e. the RV = AV /EB−V total to selective extinction ratio varies from 3 to 5.5, peaking at the densest part of L1251. The spatial structure of the head of L1251 is well modelled with a Schuster-sphere (i.e. n = 5 polytropic sphere). The observed radial distribution of mass fits the model with high accuracy out to 2.5 pc distance from the assumed center. Unexpectedly, the distribution of NH3 1.3 cm line widths is also well matched by the Schuster solution even in the tail of the cloud. Since the elongated head-tail structure of L1251 is far from the spherical symmetry the good fit of the linewidths in the tail makes it reasonable to assume that the present cloud structure has been formed by isothermal contraction.