ZCMa: the Near IR spectrum during the outburst phase

ATel #2024; S. Antoniucci (INAF - Osservatorio Astronomico di Roma), A. A.Arkharov (Central Astronomical Observatory of Pulkovo), T. Giannini (INAF - Osservatorio Astronomico di Roma), V. M. Larionov (Astronomical Institute of St. Petersburg University), D. Lorenzetti, B. Nisini (INAF - Osservatorio Astronomico di Roma)
on 20 Apr 2009; 11:08 UT
Credential Certification: Simone Antoniucci (antoniucci@oa-roma.inaf.it)

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Using the Campo Imperatore (Italy) AZT24 1m IR telescope, we obtained two near IR (0.83-2.35 um), low resolution (R~250) spectra of the variable pre-main sequence (PMS) object Z CMa, on MJD=54901 (2009 March 11) and MJD=54908 (2009 March 18). Near IR photometry in the J, H and K bands was acquired almost contemporarily (on MJD=54912): the measured magnitudes are J=5.85±0.02 mag, H=4.70±0.02 mag, K=3.56±0.02 mag. H and K band measurements were performed with narrow band filters (1.64 and 2.12 um) due to the brightness of the target.

Z CMa is a binary system (Koresko et al. 1991 AJ 102, 2073; Haas et al. 1993 A&A 269, 282) whose components are separated by 0.1" (PA=120°). The NW component is classified as an Herbig Be star (van den Ancker et al. 2004 A&A 349, 1516) and is responsible for the emerging IR emission. The SE object, dominating the visual/UV continuum, is compatible with the model of a FUOri-like source surrounded by an accretion disc (Hartmann et al. 1989 ApJ 338, 1001 ) and is also recognised as the driving source of a molecular outflow and a compact jet (Millan-Gabet & Monnier 2002 ApJ 580, L167). Recently, the two components have been resolved by long-base interferometry (Malbet et al. 1998 ApJ 507, L149; Danchi et al. 2001 ApJ 562, 440 and references therein). In January 2008, ZCMa started a still active optical outburst (Stelzer et al. 2009, arXiv:0903.4060v1 - their Fig.1), characterised by an increase of about 2 visual magnitudes. At variance with the wide optical monitoring, near-IR observations are quite rare and sparse (5-6 observations have been presented in the literature during the last 25 years) and show fluctuations up to one mag in K band. Millan-Gabet & Monnier (2002) resolved the two components indicating that in the 1986-2001 period the FUor one decreased by 1.1 mag (J band), while the Herbig Be one showed no significant flux change.

There exist only two prior near-IR spectra: on Nov 1992 by Liljestroem & Olofsson (1997 ApJ 478, 381) and on Dec 2001 by van den Ancker et al. (2004). The former shows a very marginal Brγ emission and four vibrationally excited first-overtone CO band heads in absorption. However, a quantitative comparison with our data can be done only with the latter: van den Ancker et al. measured a Brγ flux of 6e-12 erg/s/cm² and a 2.3um CO band head in absorption with an equivalent width of 2.3Å. The Table summarises the lines detected in our spectra and the measured fluxes. Being the observed line fluxes substantially equal on both spectra (taken at a temporal distance of a week from each other), we list only one value for each detected transition. Both Brγ emission and CO features are not clearly detected, so we provide only an upper limit (3σ).

Line(um)	EW(à )	F(erg/s/cm²) e-11
Paβ(1.28)	-17	1.9±0.2
Paγ(1.10)	-9	1.1±0.2
HeI(1.08)	-6	0.8±0.1
Paδ(1.01)	-2	0.3±0.1
Brγ(2.17)	>-4	<0.6
CO(2.30)	>-4	<0.6

The de-reddened (by Av=2.8 mag) Paβ/Brγ ratio (>4.9) is within the values commonly observed on young sources (e.g. Greene & Lada 1996 AJ 112, 2184). Despite the current active state of the target, our upper limit on the Brγ and CO fluxes are well compatible with the observations by van den Ancker et al., performed on Dec 2001 and corresponding to a quiescent state, which, according to the visual light curve, occurred just after a sudden and short outburst of about 1 mag. This result might indicate that the observed HI line emission is actually not related to the ongoing outburst activity (i.e. the two phenomena are presumably due to the different components).