The large amplitude outburst of the young star HBC 722 in NGC 7000/IC 5070, a new FU Orionis candidate

Abstract

Context. The investigations of the photometric and spectral variability of PMS stars are essential to a better understanding of the early phases of stellar evolution. We are carrying out a photometric monitoring program of some fields of active star formation. One of our targets is the dark cloud region between the bright nebulae NGC 7000 and IC 5070. Aims. We report the discovery of a large amplitude outburst from the young star HBC 722 (LkHα 188 G4) located in the region of NGC 7000/IC 5070. On the basis of photometric and spectroscopic observations, we argue that this outburst is of the FU Orionis type. Methods. We gathered photometric and spectroscopic observations of the object both in the pre-outburst state and during a phase of increase in its brightness. The photometric BVRI data (Johnson-Cousins system) that we present were collected from April 2009 to September 2010. To facilitate transformation from instrumental measurements to the standard system, fifteen comparison stars in the field of HBC 722 were calibrated in the BVRI bands. Optical spectra of HBC 722 were obtained with the 1.3-m telescope of Skinakas Observatory (Crete, Greece) and the 0.6-m telescope of Schiaparelli Observatory in Varese (Italy). Results. The pre-outburst photometric and spectroscopic observations of HBC 722 show both low amplitude photometric variations and an emission-line spectrum typical of T Tau stars. The observed outburst started before May 2010 and reached its maximum brightness in September 2010, with a recorded ΔV ∼ 4. m 7 amplitude. Simultaneously with the increase in brightness the color indices changed significantly and the star became appreciably bluer. The light curve of HBC 722 during the period of rise in brightness is similar to the light curves of the classical FUors -FU Ori and V1057 Cyg. The spectral observations during the time of increase in brightness showed significant changes in both the profiles and intensity of the spectral lines. Only Hα remained in emission, while the Hβ, Na I 5890/5896, Mg I triplet 5174, and Ba II 5854/6497 lines were in strong absorption.

Star formation in the “Gulf of Mexico”

“…In August 2010 the star LkHα 188 G4 = HBC 722 has increased in brightness by more than 4 magnitudes and appears to be a new FU Orionis star, as reported by Semkov et al (2010), Miller et al (2011) and Aspin et al (in prep.). The North America and Pelican nebula region, and the dark cloud that divides them, L935, in a DSS image.…”

Long-term photometric observations of pre-main sequence objects in the field of North America/Pelican Nebula

“…Magnitudes were estimated using the A to O comparison sequence of fifteen stars in the field around V2493 Cyg calibrated in BVRI by Semkov et al (2010). The used comparison sequence worked perfectly, covering the range of variability of the program stars, except for the V521 Cyg, which was brighter than comparison sequence stars.…”

“…Adding to this, observations of PMS stars are usually made difficult by their faintness in quiescence and by the disturbance from sometimes heavy dust extinction, either surrounding the star or diffuse through the star forming region. Therefore, the rare outbursts of PMS stars always attract considerable interest, especially when they are long lasting, as in the case of new FUor objects (from the name of the prototype of this class, FU Ori), with one of the most recent and spectacular cases being that of HBC 722 = V2493 Cyg (Semkov et al 2010;Miller et al 2011;Munari et al 2010;Leoni et al 2010;Kóspál et al 2011;Armond et al 2011;Green et al 2011).…”

MULTIWAVELENGTH OBSERVATIONS OF V2775 Ori, AN OUTBURSTING PROTOSTAR IN L 1641: EXPLORING THE EDGE OF THE FU ORIONIS REGIME

“…Of the other young stellar objects with both documented outbursts and FU Orionis-like near-infrared spectra (Reipurth & Aspin 2010;Semkov et al 2010;Miller et al 2011;Reipurth et al 2012), most have luminosities of at least 100 L . V2493 Cyg is the only one with a post-outburst luminosity (12 L ; Miller et al 2011) similar to or lower than the 28 L found for V2775 Ori.…”

“…Greene et al (2008) distinguish between sources in which an outburst was actually observed and those that merely have outburst-like spectra, pointing to the importance of time monitoring to unambiguously classify each putative outburster and determine the frequency of these events. Since the advent of large-scale optical and IR monitoring programs, several young stellar objects have been observed to undergo significant increases in luminosity; e.g., V733 Cep (Reipurth et al 2007), V2492 Cyg ), V2493 Cyg (Semkov et al 2010;Miller et al 2011), V2494 Cyg (Aspin et al 2009), V2495 Cyg (Movsessian et al 2006), V900 Mon (Reipurth et al 2012), and V1647 Ori (Reipurth & Aspin 2004; Abrahám et al 2004;Briceño et al 2004), allowing a more thorough understanding of the range of outburst types and their frequencies.…”