Up to September 1, 2011

ETA CARINAE  -  The most spectacular variable star of all (but very boring visually nowadays).

NOW=    Getting low in the sky around V= 4.7 as usual.
 

This is a very difficult star to observe visually. Since 1997 I have been following it and a cyclic one year pattern was recorded. Bish Ishibashi (private communication) suggested in 2003 that it might be due to airmass problems, but the fact that the comparison stars were close to the variable and of similar color seemed to rule out that effect. Also a comparison with other stars in the area revealed that they didn't suffer from noticeable extinction problems throughout the season, which made me think that the variations were real.

In a new attempt to calibrate all photometric records of this star in the same V scale, the very consistent infrared light curves by Whitelock et al. (1983, 1994, 2001, 2004) were used to construct a synthetic light curve based on a previous attempt by David Frew (private communication).
The infrared variability is similar to the visual one at least at the 0.1 mag. level, which is enough for my purpose.
The J light curve was used to fill in the gaps in the V light curve in recent years. This allowed a direct comparison between visual observations and the J trend confirming that there is a remarkable differential extinction effect causing differences up to 0.2 mag. in the visual data. The brightest magnitude being reached when the star is at its lowest as expected.
It became obvious that eta Carinae as an extended emission line object suffers from this problem in a way I never imagined.
Using the J to V calibration (with small corrections based on J-H and J-V behaviour observed in the 20 years of comparisons between V and J), the visual observations were corrected for extinction and given a common zero point with the rest of the V data.

The whole photometric history of eta Carinae was revised.
This was first done by Frew (2004) who applied corrections to minimize the diferences due to different photometric systems used, such as Stromgren, Walraven, Geneva and Johnson an different instruments and apertures.
New photometry has revealed that zero point differences are present in almost all datasets and that the amplitude of the variations is different even when trying to match contemporaneous observations.
An effort has been made to establish a mean magnitude common to all the datasets over the years, using Johnson V- band data as the starting point.
The APOG (Auckland Photoelectric Observers Group) observations are those that span longer and show a high internal consistency and thus have been used as that starting point. Most other Johnson's V datasets match rather well in this scale. OALP observations (a very consistent CCDV dataset) turn out to be 0.07 mag. brighter than the new scale adopted.
 

Recent light curve. Visual data only.

Infrared observations used for calibration:

Whitelock, P. A.; Feast, M. W.; Carter, B. S.; Roberts, G.; Glass, I. S., 1983, MNRAS, 203, 385
Whitelock, P. A.; Feast, M. W.; Koen, C.; Roberts, G.; Carter, B. S., 1994, MNRAS, 270, 364
Feast, M.; Whitelock, P.; Marang, F., 2001, MNRAS, 322, 741
Whitelock, P. A.; Feast, M. W.; Marang, F; Breedt, E., 2004, MNRAS, 352, 447
 

Light curve since the end of 1996.

PEP(V) data  from  Harry Williams & Stan Walker (Auckland Photoelectric Observers Group),  from van Genderen et al, (1999) and Sterken et al. (1999) and a couple of isolated measures from SAAO (South African Astronomical Observatory); Robert Smith and Peter McGregor (Moint Stromlo, Australia) and Mario Hamuy (Cerro Tololo Interamerican Observatory). They are quoted in Davidson et al. (AJ 118, 1777, 1999).
Whitelock et al. J data adjusted to the V scale included.

Eta Carinae historical photometry compilation:

Davidson et al., 1999, AJ, 118, 1777
Frew, D. J., 2004, The Journal of Astronomical Data, 10, 6
Sterken et al., 1999, A&A, 346, L33
van Genderen, A.M., Sterken, C., de Groot, M., Burki, G., 1999, A&A, 343, 847

A complete normalized light curve from all sources is available upon request.

For X-ray behaviour, see the updated RXTE light curve in Mike Corcoran's page: http://eud.gsfc.nasa.gov/Michael.Corcoran/eta_car/etacar_rxte_lightcurve/index.html

BVRI light curves are available at La Plata Observatory webpage: http://etacar.fcaglp.unlp.edu.ar/

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