Post-outburst phase of McNeil's nebula (V1647 Orionis)

Abstract

We present a detailed study of the post-outburst phase of McNeil's nebula (V1647 Orionis) using optical B, V, R, I and near-infrared (NIR) J, H, K photometric and low-resolution optical spectroscopic observations. The observations were carried out with the Himalaya Faint Object Spectrograph Camera (HFOSC), NIR camera (NIRCAM), the Tata Institute of Fundamental Research (TIFR) Near-Infrared Camera (TIRCAM) and NICMOS cameras on the 2-m Himalayan Chandra Telescope (HCT) and 1.2-m Physical Research Laboratory (PRL) telescopes during the period 2004 February-2005 December. The optical and NIR observations show a general decline in the brightness of the exciting source of McNeil's nebula (V1647 Ori). Our recent optical images show that V1647 Ori has faded by more than 3 mag since February 2004. McNeil's nebula has also faded considerably. The optical/NIR photometric data also show a significant variation in the magnitudes ( V = 0.78, R = 0.44, I = 0.21, J = 0.24 and H = 0.20 mag) of V1647 Ori within a period of one month, which is possibly undergoing a phase similar to eruptive variables, like EXors or FUors. The optical spectra show a few features such as strong Hα emission with blue-shifted absorption and the Ca II IR triplet (8498, 8542 and 8662 Å) in emission. As compared to the period just after outburst, there is a decrease in the depth and extent of the blue-shifted absorption component, indicating a weakening in the powerful stellar wind. The presence of the Ca II IR triplet in emission confirms that V1647 Ori is a pre-main-sequence star. The long-term, post-outburst photometric observations of V1647 Ori suggest an EXor rather than an FUor event. An optical/IR comparison of the region surrounding McNeil's nebula shows that the optical nebula is more widely and predominantly extended to the north, whereas the IR nebula is relatively confined (diameter ∼60 arcsec), but definitely extended, to the south, too. The large colour gradient from north to south and the sudden absence of an optical nebula to the south are suggestive of a large-scale disc-like structure (or envelope) surrounding the central source that hides the southern nebula.

REAPPEARANCE OF McNEIL'S NEBULA (V1647 ORIONIS) AND ITS OUTBURST ENVIRONMENT

“…Since OI λ7773Å cannot be formed in photosphere of cool stars, the absorption component is due to warm gas in the envelope or hot photosphere above disc, while the emission component might be due to the hot gas region from which Hα is also being emitted. Ojha et al (2006) reported a decreasing trend in the W λ of OI λ7773Å from the beginning till end of the 2003 outburst which was interpreted to be possible decrease in turbulence in outer envelope during the outburst period. Ojha et al (2006), in contrast to the increase of W λ of other lines, could be due to increase in the W λ of OI λ7773Å emission component.…”

“…Ojha et al (2006) reported a decreasing trend in the W λ of OI λ7773Å from the beginning till end of the 2003 outburst which was interpreted to be possible decrease in turbulence in outer envelope during the outburst period. Ojha et al (2006), in contrast to the increase of W λ of other lines, could be due to increase in the W λ of OI λ7773Å emission component. …”

“…Data reduction and final photometry were done using standard IRAF aperture photometric tasks. To be consistent with magnitude estimates by Ojha et al (2006), for flux calibration we used an aperture ∼ 7 arcsec, and for background sky estimation we used an annulus with an inner radius of ∼ 50 and width ∼ 5 . For instrumental to apparent magnitude calibration, we observed standard stars around AS13, AS9 and HD225023 fields (Hunt et al 1998) on the same night with similar airmass as V1647 Ori observations.…”

“…They also reported that the mid-infrared emitting region at 10 µm has a size of ∼ 7 AU. Rettig et al (2005) used the CO lines in infrared (IR) to measure the temperature of the inner accretion disc region which was estimated to be T ≈ 2500 K. Ojha et al (2006) and Kóspál et al (2005) reported a sudden dimming and termination of the 2003 outburst in November 2005. Thus, 2003 outburst lasted for a total of ∼ 2 years and V1647 Ori returned to its pre-outburst phase in early 2006.…”

“…This method reduced our error bars in magnitude by a factor of 2. Apart from the Gaussian convolution step, the PSF photometry steps were all same as Ojha et al (2006).…”