ABSTRACT: Context: It is hypothesized that low-mass young stellar objects undergo
eruptive phases during their early evolution. The outburst of V1647 Ori between
2003 and 2006 offered a rare opportunity to investigate such an accretion
event. Aims: By means of our interferometry observing campaign during this
outburst, supplemented by other observations, we investigate the temporal
evolution of the inner circumstellar structure of V1647 Ori We also study the
role of the changing extinction in the brightening of the object and separate
it from the accretional brightening. Methods: We observed V1647 Ori with
MIDI/VLTI at two epochs in this outburst. First, during the slowly fading
plateau phase (2005 March) and second, just before the rapid fading of the
object (2005 September), which ended the outburst. We used the radiative
transfer code MC3D to fit the interferometry data and the spectral energy
distributions from five different epochs at different stages of the outburst.
The comparison of these models allowed us to trace structural changes in the
system on AU-scales. We also considered qualitative alternatives for the
interpretation of our data. Results: We found that the disk and the envelope
are similar to those of non-eruptive young stars and that the accretion rate
varied during the outburst. We also found evidence for the increase of the
inner radii of the circumstellar disk and envelope at the beginning of the
outburst. Furthermore, the change of the interferometric visibilities indicates
structural changes in the circumstellar material. We test a few scenarios to
interpret these data. We also speculate that the changes are caused by the
fading of the central source, which is not immediately followed by the fading
of the outer regions. However, if the delay in the fading of the disk is
responsible for the changes seen in the MIDI data, the effect should be
confirmed by dynamical modeling.
ABSTRACT: 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
ABSTRACT: 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 um
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 um
low-resolution spectra obtained with the IRS 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 rate.
In several systems, all exhibiting 10 um silicate emission, the variability of
the 6-8 um 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, are more
ubiquitous than was known before. Interpreting this variability is a new
possibility to explore the structure of the disk and its dynamical processes.
ABSTRACT: V1647 Ori is a young eruptive variable star, illuminating a reflection nebula
(McNeil's Nebula). It underwent an outburst in 2003 before fading back to its
pre-outburst brightness in 2006. In 2008, V1647 Ori underwent a new outburst.
The observed properties of the 2003-2006 event are different in several
respects from both the EXor and FUor type outbursts, and suggest that this star
might represent a new class of eruptive young stars, younger and more deeply
embedded than EXors, and exhibiting variations on shorter time scales than
FUors. In outburst, the star lights up the otherwise invisible McNeil's nebular
- a conical cloud likely accumulated from previous outbursts. We present
follow-up photometric as well as optical and near-IR spectroscopy of the nebula
obtainted during the 2008-2009 outburst. We will also present results from
contemporaneous X-ray observations. These multi-wavelength observations of
V1647 Ori, obtained at this key early stage of the outburst, provide a snapshot
of the "lighting up" of the nebula, probe its evolution through the event, and
enable comparison with the 2003-2006 outburst.
ABSTRACT: Young Sun-like stars at the beginning of the pre-main-sequence (PMS) evolution are surrounded by accretion discs and remnant protostellar envelopes. Photometric and spectroscopic variations of these stars are driven by interactions of the star with the disc. Time-scales and wavelength dependence of the variability carry information on the physical mechanisms behind these interactions. We conducted multi-epoch, multiwavelength study of PV Cep, a strongly variable, accreting PMS star. By combining our own observations from 2004 to 2010 with archival and literature data, we show that PV Cep started a spectacular fading in 2005, reaching an IC-band amplitude of 4 mag. Analysis of variation of the optical and infrared fluxes, colour indices and emission line fluxes suggests that the photometric decline in 2005–2009 resulted from an interplay between variable accretion and circumstellar extinction: since the central luminosity of the system is dominated by accretion, a modest drop in the accretion rate could induce the drastic restructuring of the inner disc. Dust condensation in the inner disc region might have resulted in the enhancement of the circumstellar extinction.
Monthly Notices of the Royal Astronomical Society 05/2011; 413(4):2689 - 2695. · 4.90 Impact Factor
ABSTRACT: 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 C band) brightness variations on 2-3 years timescales, flat spectral energy distribution (SED), and extremely strong (300 EW/Å 900) Hα 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.
The Astrophysical Journal Letters 04/2011; 733(1):L8. · 5.53 Impact Factor
ABSTRACT: Context. In August 2010, the sudden optical brightening of two young stellar
objects, located in the North America/Pelican Nebula Complex, was announced.
Early observations indicated that these objects may belong to the FUor or EXor
class of young eruptive stars. The eruptions of FUors and EXors are often
explained 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, and 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 color-color diagrams. Results. HBC 722
brightened 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, questioning its classification as a bone fide FUor.
The outburst of VSX J205126.1+440523 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 monitoring indicates that it is still far from being
quiescent. Conclusions. The shape of the light curves, the bolometric
luminosities and accretion rates suggest that these objects do not fit into the
classic FUor group. Although HBC 722 exhibit all spectral characteristics of a
bona fide FUor, its luminosity and accretion rate is too low, and its timescale
is too fast compared to classical FUors. VSX J205126.1+440523 seems to be an
example where quick extinction changes modulate the light curve.
ABSTRACT: 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 Å 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 Astrophysical Journal 03/2009; 695(1):619. · 6.02 Impact Factor
ABSTRACT: The northern Milky Way in the constellation of Cepheus (100 deg < l < 120 deg; 0 deg < b < 20 deg) contains several star forming regions. The molecular clouds of the Cepheus Flare region at b > 10 deg, are sites of low and intermediate mass star formation located between 200 and 450 pc from the Sun. Three nearby OB associations, Cep OB2, Cep OB3, Cep OB4, located at 600-800 pc, are each involved in forming stars, like the well known high mass star forming region S140 at 900 pc. The reflection nebula NGC 7129 around 1 kpc harbors young, compact clusters of low and intermediate mass stars. The giant star forming complex NGC 7538 and the young open cluster NGC 7380, associated with the Perseus arm, are located at d > 2 kpc.
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 12CO and 13CO observations of the cloud. We determined the effective temperatures and luminosities of 14 pre-main sequence stars associated with the cloud from their positions in the Hertzsprung--Russell diagram, as well as constructed their spectral energy distributions using optical, 2MASS and Spitzer IRAC and MIPS data. We derived physical parameters of L1622 from the molecular observations. Our results are 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 pc the mass of the cloud, derived from our CO data, is 1100 solar masses, its star formation efficiency is 1.8%, and the average age of its low-mass pre-main sequence star population is about 1 million years. Comment: 13 pages, 9 figures, 4 tables; accepted by MNRAS
ABSTRACT: We report on new optical spectroscopic observations of the young
eruptive star EX Lupi. In 2008 January-February, the star reached the
highest peak brightness ever observed. The outburst spectra differ
significantly from previous ones, displaying many strong metallic
Information Bulletin on Variable Stars. 02/2008; 5819:1.
ABSTRACT: We studied the brightness and spectral evolution of the young eruptive star V1647 Ori during its recent outburst in the period 2004 February-2006 September. We performed a photometric follow-up in the bands V, RC, IC, J, H, and Ks, as well as visible and near-IR spectroscopy. The main results derived from combining our data with those published by other authors are as follows: the brightness of V1647 Ori stayed more than 4 mag above the preoutburst level until 2005 October, when it started a rapid fading. In the high state we found a periodic component in the optical light curves with a period of 56 days. The delay between variations of the star and variations in the brightness of clumps of nearby nebulosity corresponds to an angle of 61° ± 14° between the axis of the nebula and the line of sight. The overall appearance of the infrared and optical spectra did not change in the period 2004 March-2005 March, although a steady decrease of H I emission-line fluxes could be observed. In 2006 May, in the quiescent phase, the He I 1.083 μm line was observed in emission, contrary to its deep blueshifted absorption observed during the outburst. The J - H and H - Ks color maps of the infrared nebula reveal an envelope around the star whose largest extension is about 18'' (0.03 pc). The color distribution of the infrared nebula suggests reddening of the scattered light inside a thick circumstellar disk. Comparison of the Ks and Hα images of McNeil's Nebula, the conical nebulosity illuminated by V1647 Ori, shows that HH 22A, the Spitzer infrared source, and the bright clump C of the nebula may be unrelated objects. We show that the observed properties of V1647 Ori could be interpreted in the framework of the thermal instability models of Bell and coworkers. V1647 Ori might belong to a new class of young eruptive stars, defined by relatively short timescales, recurrent outbursts, a modest increase in bolometric luminosity and accretion rate, and an evolutionary state earlier than that of typical EXors.
The Astronomical Journal 12/2007; 133(5):2020. · 4.03 Impact Factor
ABSTRACT: We determined [Fe/H] for 17 double-mode Cepheids of the southern hemisphere using high-resolution echelle spectra. For five
of the program stars these are the first iron abundances in the literature. The available data suggest that the period ratio
for galactic beat Cepheids is mainly determined by their metallicity. Therefore, the observed period ratios do not reflect
large differences in their mass, effective temperature or luminosity.
11/2007: pages 169-172;
ABSTRACT: From a long time series of high resolution (R=115,000) HARPS spectra and simultaneous broad-band photometry, we report new evidence for magnetospheric accretion as well as ejection processes in the nearly edge-on classical T Tauri star AA Tau. AA Tau's light curve is modulated with a period of 8.22d. The recurrent luminosity dips are due to the periodic occultation of the central star by the magnetically-warped inner disk edge located at about 9 Rstar. Balmer line profiles exhibit a clear rotational modulation of high-velocity redshifted absorption components with a period of 8.22 days as well, with a maximum strength when the main accretion funnel flow passes through the line of sight. At the same time, the luminosity of the system decreases by about 1 mag, indicative of circumstellar absorption of the stellar photosphere by the magnetically-warped, corotating inner disk edge. The photospheric and HeI radial velocities also exhibit periodic variations, and the veiling is modulated by the appearance of the accretion shock at the bottom of the accretion funnel. Diagnostics of hot winds and their temporal behaviour are also presented. The peculiar geometry of the young AA Tau system (nearly edge-on) allows us to uniquely probe the acretion-ejection region close to the star. We find that most spectral and photometric diagnostics vary as expected from models of magnetically-channelled accretion in young stars, with a large scale magnetosphere tilted by 20 deg onto the star's spin axis. We also find evidence for time variability of the magnetospheric accretion flow on a timescale of a few rotational periods. Comment: 14 pages
ABSTRACT: Medium-resolution optical spectroscopy of the candidate young stellar objects associated with the small, nearby molecular cloud Lynds 1333 revealed four previously unknown classical T Tauri stars, two of which are components of a visual double, and a Class I source, IRAS 02086+7600. The spectroscopic data, together with new V, RC, IC photometric and 2MASS J, H, and Ks data allowed us to estimate the masses and ages of the new T Tauri stars. We touch on the possible scenario of star formation in the region. L 1333 is one of the smallest and nearest known star-forming clouds, therefore it may be a suitable target for studying in detail the small-scale structure of a star-forming environment.
Monthly Notices of the Royal Astronomical Society 09/2006; 371(2):732 - 738. · 4.90 Impact Factor
ABSTRACT: New BV RI photometry and optical spectroscopy of the Type IIp supernova 2004dj in NGC 2403, obtained during the first year since discovery, are presented. The progenitor cluster, Sandage 96, is also detected on pre-explosion frames. The light curve indicates that the explosion occurred about 30 d before discovery, and the plateau phase lasted about +110 ± 20 d after that. The plateau-phase spectra have been modelled with the synow spectral synthesis code using H, Na i, Ti ii, Sc ii, Fe ii and Ba i lines. The SN distance is inferred from the expanding photosphere method and the standard candle method applicable for SNe IIp. They resulted in distances that are consistent with each other as well as earlier Cepheid and Tully–Fisher distances. The average distance, D= 3.47 ± 0.29 Mpc is proposed for SN 2004dj and NGC 2403. The nickel mass produced by the explosion is estimated as ∼0.02 ± 0.01 M⊙. The spectral energy distribution of the progenitor cluster is reanalysed by fitting population synthesis models to our observed BV RI data supplemented by U and JHK magnitudes from the literature. The χ2 minimization revealed a possible ‘young’ solution with cluster age Tcl= 8 Myr, and an ‘old’ solution with Tcl= 20–30 Myr. The ‘young’ solution would imply a progenitor mass M > 20 M⊙, which is higher than the previously detected progenitor masses for Type II SNe.
Monthly Notices of the Royal Astronomical Society 07/2006; 369(4):1780 - 1796. · 4.90 Impact Factor
ABSTRACT: V1647 Ori is a young eruptive star, which went into outburst at the end of 2003. Since then, the object is gradually fading. In October 2005, however, V1647 Ori started a very rapid fading, approximately 1 mag/month in I_C as opposed to 0.1 mag/month previously. According to the colour changes, the rapid fading is not caused by increasing extinction, which suggests that the observed brightness changes are intrinsic. Supposing that the fading rate remains unchanged, the star will return to the pre-outburst state by mid-December 2005.
ABSTRACT: The Cepheus-Cassiopeia star-forming region has been searched for new T Tauri stars (TTSs) based on the ROSAT all sky survey (RASS). Optical spectroscopic observations were carried out toward 45 GSC stellar counterparts of RASS sources looking for the Li absorption line (6708 Å), a sign of youth. The detection of this line resulted in finding 11 to 16 new TTSs in this region. Using follow-up optical photometric observations and evolutionary models these Li-rich stars are revealed to be young low-mass stars. The most of the young stars are separated from the 13 CO molecular clouds by ∼10 pc, significantly more than in Chamaeleon. A group of the new TTSs are isolated from the CO clouds and distributed inside the previously determined CO void. Possible formation scenarios including the interaction with the supernova shock for the isolated TTSs are discussed.
A&A. 01/2005; 437:919-928.
ABSTRACT: We combine our V , R, I , J , H , K s photometric and near infrared spectroscopic observations of the outburst star V1647 Ori obtained in the period 12 February–24 April 2004 with published data in order to make inferences on the geometry of the object and the nature of the eruption. The main results are as follows: (1) Our photometric results do not indicate systematic fading of the star in the period of observations. (2) Assuming a spectral type A0 in the visual wavelength regime, the observed Rc − Ic colour index of the star yields a reddening AV ≈ 8.4 mag, and with this AV the mean observed V ≈ 17.4 mag corresponds to MV ≈ 1.0 mag. (3) Examination of the time delay between the brightness variations of the star and two nebular positions allowed us to derive an angle of about 60 • between the axis of the nebula and the line of sight. (4) Comparison of the K s and Rc images of McNeil's nebula shows that HH22A consists of two physically different knots: the southern is a part of the nebula but not seen in the Ks, while the northern one is outside the illuminated region, but exhibits H2 λ2.12µm emission. (5) The J − H and H − Ks colour maps of the infrared nebula, which trace the density structure of the circumstellar environment of V1647 Ori, reveal a flattened envelope whose main axis, about 20 ′′ (0.04 pc) in size, is perpendicular to the axis of the optical nebula. (6) As an explanation for the lack of shock signatures in the spectrum of V1647 Ori, we propose that, instead of dust removal from the line of sight to the star by stellar winds, the appearance of a new, hot source, exhibiting the observed B–A type spectrum, accounts for both the illumination of the nebula and the colour and brightness variations of the star. (7) No clear evidence of magnetospheric accretion, characteristic of EXors, can be identified in the published optical and near infrared spectra of the star. On the contrary, several features characteristic of FUors do appear. Together with the light curve and SED the spectroscopic properties suggest that V1647 Ori is an FU Ori type star.
ABSTRACT: (Abridged) We studied the brightness and spectral evolution of the young eruptive star V1647 Ori during its recent outburst in the period 2004 February - 2006 Sep. We performed a photometric follow-up in the bands V, R_C, I_C, J, H, K_s as well as visible and near-IR spectroscopy. The main results are as follows: The brightness of V1647 Ori stayed more than 4 mag above the pre-outburst level until 2005 October when it started a rapid fading. In the high state we found a periodic component in the optical light curves with a period of 56 days. The delay between variations of the star and variations in the brightness of clump of nearby nebulosity corresponds to an angle of 61+/-14 degrees between the axis of the nebula and the line of sight. A steady decrease of HI emission line fluxes could be observed. In 2006 May, in the quiescent phase, the HeI 1.083 line was observed in emission, contrary to its deep blueshifted absorption observed during the outburst. The J-H and H-K_s color maps of the infrared nebula reveal an envelope around the star. The color distribution of the infrared nebula suggests reddening of the scattered light inside a thick circumstellar disk. We show that the observed properties of V1647 Ori could be interpreted in the framework of the thermal instability models of Bell et al. (1995). V1647 Ori might belong to a new class of young eruptive stars, defined by relatively short timescales, recurrent outbursts, modest increase in bolometric luminosity and accretion rate, and an evolutionary state earlier than that of typical EXors.