Photometry of the Inner Eta Carinae Nebula

As a by-product of the Hubble Space Telescope Eta Carinae Treasury Project observing campaign, every observation with the HST STIS has included an "acquisition image" of the inner part of the Eta Carinae nebula. Nominally, the acquisition image used to point the HST and center the target on the spectrograph. The images are saved in the HST archive as uncalibrated raw output which is not conducive to further processing. Yet these images represent a high resolution record of changes in the inner three arc seconds of the Eta Carinae nebula, which cannot be resolved from the ground. Considering that this is the inner 1000 AU of the nebula (roughly 100 times the size of our solar system) we should expectto see changes that proceed at least on the time scale of the 5.5 year spectroscopic cycle if not shorter.

A typicalSTIS acquisition image (which is about 3.5x3.5 arcseconds, or about the size of a typical ground-based seeing disk) shows that the central part of the Eta Carinae nebula is much more complex than a single central point source.

Myself and Michael Koppleman reduced the raw data from the acquisition images, photometrically calibrated them, and then extracted photometric information about changes in the inner 0.3 arcseconds of the nebula. We found that the central star of Eta Carinae leveled off in brightness in early 2000 but then started to again brighten dramatically about six months before the 2003.55 spectroscopic event. The evidence supports that this is a change in continuum brightness probably caused by clearing of obscuring material around the central star.

Here is a copy of the poster presented at the January 2004 AAS meeting.

Here is a copy of the pre-print for our accepted publication to appear in the April 2004 Astronomical Journal.

The STIS/CCD ACQ Data

There are more observations of Eta Carinae planned for the future. As these observations are made we will add the points to the plot below. The red crosses are the raw measurements and the green triangles are the flux minus the contribution from H\alpha. The black arrows mark the times of the 1998.0 and 2003.5 spectroscopic events.

March 2004 The latest observations indicate that the brightening of the central star may have leveled off sometime between the November 2003 and March 2004. The 4% difference in flux between the November 2003 and March 2004 observations are not significant considering the accuracy of the absolute flux calibration of the STIS/CCD. It is tempting to draw further conclusions from this one point but we will resist that temptation at this time especially since we have no observation at the same "phase" after the 1998.0 spectroscopic event to compare this point to.

The ACS/HRC Data

The treasury project and other observing programs have made multiple observations of Eta Carinae from late 2002 through the present day. Observations with the ACS/HRC have been made primarily in four filters: F550M, F330W, F250W, and F220W.

The F220W and F250W filters measure the NUV flux, which is affected by extreme line blanketing during the spectroscopic events. The F330W filters measures the changes in the Balmer jump, which is an emission feature in the spectrum of Eta Carinae. The F550M filter is mostly devoid of emission features in Eta's spectrum so that this is a measure of the continuum brightness of thecentral star. Note in the plot below that the continuum brightness did not dip appreciably during the 2003.5 spectroscopic event (dotted vertical line). This is because the changes in brightness measured in broad-band filters are probably completely affected by changes in the spectral emission features. The increase in the brightness of the continuum is a trend which started in the 1950's and has continuued to the present day. Since 1998, we have been in an epoch where the central star has brightened at an accelerated rate.

March 2005 We added data acquired by the ACS/HRC in late December 2004. The data show that the relative flux in each filter has restored approximately to the level before the 2003.5 spectroscopic event. So the central star has recovered completely from the event 15 months ago. The accelerated brightening rate has continued. Note that in 2004 the continuum brightness (F550M filter) increased by about 0.2 magnitudes/year. This is a slower rate than observed in 2003 (about 0.5 magnitudes/year) but is still a faster rate than observed in the longterm trend over the last 50 years (0.01 magnitudes/year).

August 2005 Data from July 2005 were added. The brightening rate appears to have leveled off completely in the F550M filter but continues to increase in the other filters. This could be interpreted to mean that the continuum brightness has leveled out but the emission features in the spectrum continue to grow in flux.

Planned Future Observations:

For up to date information on planning of the HST Eta Car Treasury Project observations see http://presto.stsci.edu/cgi-bin/get-visit-status?9973. As of September 2004, the STIS is broken and chances for repair look dim so it is more than likely that no more points will be added to the STIS/ACQ plot. The remaining orbits on the program will be used to gather images using the ACS/HRC so that plot will be updated as those observations are made.

Last updated 23 August 2005