[vsnet-chat 4658] Re: [vsnet-obs 34771] eta Car

[Bish Ishibashi <bish@howdy.gsfc.nasa.gov>]

Dear Sebastian and Berto:


Indeed the brightness of Eta Carinae itself has declined, 
according to our monitoring of the central source with 
the Hubble Space Telescope. We use an acquisition image 
of Eta Carinae taken with Space Telescope Imaging Spectro-
graph to do spatially resolved photometry (not to be 
confused with the famous WFPC2 image of Eta Carinae; 
we don't do that here). 

At each time the HST/STIS visits Eta, it takes a snapshot
image of Eta Carinae to pin-point its bore-sight. The snap
shot image is 1024 x 1024 pixels in size with 1 pixel = 
0.0507 arc-seconds. No filter is used if my memory serves. 

Ted Gull, one of two team leaders of our group, has been 
performing a simple photometry on each acquisition image
taken with the STIS. Two measurements are done with an 
aperture centered on the central bright source and with 
a square aperture array 5 x 5 and 96 x 96 pixels (i.e., 
0.25 x 0.25 and 4.9 x 4.9 square-arc-seconds). The results
are tabulated below: 


 DATE		5x5(star) ratio	96x96     ratio	
Dec 97		24187 c/s 1.00	96035 c/s 1.00	
Mar 98		27849	  1.15	141500	  1.47	
Nov 98		39000	  1.61	173165	  1.80	
Feb 99		43002	  1.78	261620	  2.72	
Mar 13 2000	53758	  2.22	382175	  3.98	
Mar 20 2000	54795	  2.27	385880	  4.02	
Oct  9 2000	49324	  2.04	256692	  2.67	
Apr 17 2001	48204	  1.99	282394	  1.98	

Each measurement gives the count rate detected in the 
selected aperture. For your convenience, the intensity 
is normalized with the value taken in December 1997 and 
also tabulated (see "ratio").  As shown, the rates have 
declined in both cases. 

However, take a look at the difference of these two 
measurements (i.e., count_rate(96x96) - count_rate(5x5)). 


 DATE		Delta (Nebula)  ratio
Dec 97		71848 c/s	1.00
Mar 98		113651		1.58
Nov 98		134165		1.87
Feb 99		218618		3.04
Mar 13 2000	318418		4.43
Mar 20 2000	331085		4.69
Oct  9 2000	207368		2.89
Apr 17 2001	234190		3.26


This result indicates that the nebula surrounding the 
central bright source has not followed the same trend
as the star exhibited lately. Likely that we need to take
light-travel time within the Homunculus nebula into 
account to explain these variations. Note that our 
acquisition image does not cover the entire Homunculus
and therefore we do not know precisely what part of 
the Homunculus contributes most to photometry measurements
that Sebastian referred to. 


One of our team members, Nathan Smith, has worked out 
the same with the STIS spectroscopic dataset and show
the same declining trend through visual to Near IR 
wavelengths on the star. As far as I know he has submitted
his work to a journal for publication, though the detail
I have not heard of and I won't comment on it further. 


.....and there is one cautionary remark to Sebastian. 
When people say "S Dor phase" it means a very specific
phenomenon. When a star undergoes S Dor phase, it is 
said that the star shows a cooler spectrum and colors
(i.e., the peak of the continuum shifts toward red 
wavelengths) as the star itself brightens. This is 
what one expects in a normal LBV-style visual brightening
event. But that's not what has been going on with Eta Car. 
As Eta brightened last time, its spectrum was known to become
hotter and bluer. Nathan Smith has a figure to demonstrate
this, but that's his work and it's under review by a 
journal. So let me not refer to it but instead to refer
to one of the old figures used in my Ph.D. thesis (gasp!):  


	http://hires.gsfc.nasa.gov/~bish/EC/FIG53.gif


Never mind about the large amplitude fluctuation in the
ratio plot around 2500 angstrom. That's due to Fe II 
emission forest changing dramatically during and after
the last 5.5-yr event of Eta Carinae. The point is that 
the ratio values are generally higher in blue wavelengths
than that in red. So by definition, Eta Car's last 
brightening was not a S Dor phenomenon. We made clear 
this point in the publication by Davidson et al. (2000) 
in Astrophysical Journal. If you don't have access to 
such via ADS or via a local library, then go to the 
following site of mine to grab the original manuscript:


	http://hires.gsfc.nasa.gov/~bish/reshst.html


and click "An Unusual Brightening of Eta Car..." link
(either postscript or pdf) to download the paper. 


That said, I agree more on what Berto Monard said in 
the exchange, as far as the trend in brightness of the
central star goes. It's remotely possible that the Homunculus 
may be still brightening in visual wavelengths a bit (which 
I cannot confirm here), but that's hard to say from the limited 
data that we have here. 


As my usual ending, I do not wish to discourage anyone from 
observing this difficult target whenever you can. I'd buy a CCD 
camera to do photometry every clear night (and Eta's up) if I ever 
lived in the southern hemisphere. But somehow I always end
up with a job in the northern hemisphere.

Bish Ishibashi


-- 
----------------------------------------------------------------------
   Kazunori "Bish" Ishibashi(NRC/NAS)  E: bish@howdy.gsfc.nasa.gov   
   NASA/GSFC Code 681                  W: 1-301-286-8904
   Greenbelt, MD 20771                 F: 1-301-286-1752
   http://hires.gsfc.nasa.gov/~bish/
----------------------------------------------------------------------

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp