The 2008 outburst in the young stellar system Z CMa

Abstract

Context. Z CMa is a complex pre-main sequence binary with a current separation of 110 mas, known to consist of an FU Orionis star (SE component) and an embedded Herbig Be star (NW component). Although it represents a well-studied and characterized system, the origin of photometric variabilities, the component properties, and the physical configuration of the system remain mostly unknown. Aims. Immediately when the late-2008 outburst of Z CMa was announced to the community, we initiated a high angular resolution imaging campaign aimed at characterizing the outburst state of both components of the system in the near-infrared. Methods. We used the VLT/NACO and the Keck/NIRC2 near-infrared adaptive optics instrument to monitor the astrometric position and the near-infrared photometry of the Z CMa components during the outburst phase and one year after. The VLT/SINFONI and Keck/OSIRIS integral field spectroscrographs were in addition used to characterize for the first time the resolved spectral properties of the FU Orionis and the Herbig Be component during and after the outburst. Results. We confirm that the NW star dominates the system flux in the 1.1-3.8 µm range and is responsible for the photometric outburst. We extract the first medium-resolution (R∼2000-4000) near-infrared (1.1-2.4 µm) spectra of the individual components. The SE component has a spectrum typical of FU Orionis objects. The NW component spectrum is characteristic of embedded outbursting protostars and EX Or objects. It displays numerous emission lines whose intensity correlates with the system activity. In particular, we find a correlation between the Brγ equivalent width and the system brightness. The bluing of the continuum of the NW component along with the absolute flux and color-variation of the system during the outburst suggests that the outburst was caused by a complex interplay between a variation of the extinction in the line of sight of the NW component on one hand, and the emission of shocked regions close to the NW component on the other. We confirm the recently reported wiggling of the SE component jet from [FeII] line emission. We find a point-like structure associated with a peak emission at 2.098µm coincidental with the clump or arm seen in broadband polarization differential imaging as well as additional diffuse emission along a PA=214• . The origin of these two structures is unclear and deserves further investigation.

Sub-0.1′′ optical imaging of the Z CMa jets with SPHERE/ZIMPOL

“…), we obtain a separation of 114.2 ± 3.1 mas and a position angle of 136.9 ± 1.5 deg, which corresponds to a projected length of about 131 AU. This measurement and others collected in a time span of 26 yr (Koresko et al 1991;Haas et al 1993;Thiebaut et al 1995;Barth et al 1994;Millan-Gabet et al 2002;Bonnefoy et al 2016) are reported in Fig. A.1.…”

“…The two sources, located at a distance of 1150 pc (Herbst et al 1978), are separated by 0 . 1 and only recently high angular resolution observations with adaptive optics (AO) systems and interferometers have allowed the separatele determination of the properties of the two sources (Bonnefoy et al 2016;Hinkley et al 2013). The Herbig star, which is responsible for the variability of the binary (Bonnefoy et al 2016), has shown recurrent variations in the visual magnitude of 2−3 mag in the last ten years with recent major events recorded in , 2011(see light curves in Canovas et al 2012Maehara & Ukita 2015).…”

“…1 and only recently high angular resolution observations with adaptive optics (AO) systems and interferometers have allowed the separatele determination of the properties of the two sources (Bonnefoy et al 2016;Hinkley et al 2013). The Herbig star, which is responsible for the variability of the binary (Bonnefoy et al 2016), has shown recurrent variations in the visual magnitude of 2−3 mag in the last ten years with recent major events recorded in , 2011(see light curves in Canovas et al 2012Maehara & Ukita 2015). These variations have been imputed to both variable accretion and to changes of the extinction along the line of sight (Benisty et al 2010;Szeifert et al 2010).…”

The 2014–2017 outburst of the young star ASASSN-13db

“…While the first outburst is in full agreement with typical EXor outbursts (e.g., Herbig et al 2001), the second one is unusual in length and in magnitude. The length of the 2014-2017 outburst is shorter than typical FUors, although several FUor outbursts lasting only a few years have been observed (e.g., ZCMa, V1647 Ori, [CTF93]216-2; Fedele et al 2007;Aspin 2011;Caratti o Garatti et al 2011;Audard et al 2014;Bonnefoy et al 2016). Objects with similar outburst lengths are often referred to as EXors, but this is typically accompanied by a question mark or considered as an intermediate case, especially if some FUor characteristics are present (e.g., Ábrahám et al 2004;Fedele et al 2007;Caratti o Garatti et al 2011).…”

Near-infrared High-resolution Imaging Polarimetry of FU Ori-type Objects: Toward a Unified Scheme for Low-mass Protostellar Evolution*

“…We are concerned that the Gaia measurements in 2014-2016 might have suffered significantly from photometric variabilities of this binary system with a d = 0. 11 separation (Bonnefoy et al 2017). We therefore assume d=1000 pc based on Kaltcheva & Hilditch (2000).…”