Carbon Star - V Hydrae
In the three dimensional context of the placement of all stars and their individual motions relative to us here on Earth, there exists a broad correlation between distance and proper motion. Generally speaking, the further away a star is located from us in distance, the smaller will be its observed proper motion. A proper motion of ~14 milliarcseconds per year for V Hydrae suggests the star cannot be much nearer than 1,000 light years away from us.
V Hydrae is to be found in the springtime sky, low in the south by late evenings during April and May.
The brightness of V Hydrae normally fluctuates between magnitudes 6.5 and 9.5 over a period of about 530 days. However, every 18 years or so the star undergoes a deep minima that can take it down to below 12th magnitude. My regression model fitted to historical AAVSO data (below) shows the underlying average brightness of this star to have faded from mag 7.9 to 8.9 between 1913 (earliest) and 2010 (latest). This is also reflected in fainter peaks and fainter troughs, with a deep minimum to mag 13.4 persisting between 1993 and 1994:
Another minimum is due anytime!
As I write (May 2010), V Hydrae is due for another one of its deep, 18-year cyclical plunges going into 2011. I wonder how faint it will go this time round? The likelihood is this aged star has already began to shed its outer layers as it heads toward a fate of turning into a white dwarf, which will be eventually surrounded by a planetary nebula.This is indeed an exciting time to study this star, though its steep southerly declination makes poor viewing from where I'm based in the British Isles.
How to find V Hydrae. The star is just a short hop down from the 4th magnitude stars Alkes (Alpha) and Beta Crateris. Stars are shown down to magnitude 10.
What causes these 18-year plunges in brightness?
My own theory is that the material peeling off of V Hydrae is clumping together into a debris and dust disk around the star, which is in an 18 year orbit. Every 18 years, the densest part of this debris disk passes in front of the star, blotting out some of its light and causing its brightness to fade in the manner that it does. This appears to be consistent with the observed regularity of the 18 year brightness plunges. Furthermore, the dust disk is probably of variable density and somewhat irregular in shape... which means the amount of light it blots out will not be *exactly* the same each orbit round. Why would the material clump together in this way? Well, suppose the debris disk shares its existence with a gas giant planet in that same 18 year orbital path around V Hydrae. The gas giant will then sweep up material from the disk and concentrate it around itself, as it orbits the star. This would then explain the oscillatory nature of the pre and post-minima lightcurve we see in the steep 18-year fades of this highly remarkable variable star.Astronomical Observations & Research home page
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