MBCAA Observatory

V1118 Ori eruption: RVB observations

Observed 10 Jan 2005

Michel Bonnardeau
7 Feb 2005
Revised: 31 March, 24 April, 5 May 2005

Abstract

The eruptive star V1118 Ori was observed during an outburst as fading down and cooling down.

Introduction

The eruptive variable V1118 Ori was reported in outburst on 10 Jan 2005, for the first time since 1997 (Williams 2005). I observed it from 19h TU to 01h TU with Rc, V, B filters.

Observations

Time-series observations were carried out with a 203 mm SC telescope, a SBIG ST7E camera (KAF401E CCD chip) and Johnson-Cousins Rc, V, B filters in a filter wheel. 31 images were obtained with the Rc filter, 29 with the V filter and also 29 with the B filter. Each exposure is 200-second long.

The differential photometry is somewhat challenging: nebulosities from the Orion Nebula M42 are present and nearly all the stars in the field are known variables.
The comparison star is UCAC2 29776568 with a R magnitude of 11.73.
The check star is GSC 4774-00825. It is observed to have the average relative magnitudes Rc=0.242+/-0.023, V=0.081+/-0.022, B=-0.272+/-0.045 where the errors are the standard deviations.

With the Rc filter. The comparison star has the yellow symbol. The apertures are also shown. The ring of the comparison star aperture is large so as not to include a nearby star.

The light curves are:

The error bars are the +/- 1 sigma statistical uncertainties. The dot lines are best fits.

The light curves for the check star and the air mass variation are here.

Analysis

The variable is observed as decaying. The decay slopes are not the same for the different bands, with the B light curve being steeper than the V, itself steeper than the Rc:
R band: 0.74 mag/day
V band: 0.97 mag/day
B band: 1.25 mag/day
The uncertainty is about 0.06. This reddening is not observed for the check star, therefore it is intrinsic to the variable star and not due to the air mass increase during the session. This suggests that V1118 is cooling down as it is fading down.