The Disappearing Nebula - PV Cephei
RA: 20h 45m 55.5sec Size: 30"
DEC: +67° 57'45" m = +15 (star), + 14 (nebula)
In the mid 1970's astronomer Martin Cohen began a systematic study of the entire National Geographic POSS (Palomar Observatory Sky Survey) plates in a search for faint nebulous objects that may be relevant to pre-main sequence evolution. In December 1976 a new plate was taken with the Lick Observatory 3 meter reflector of an object Cohen had previously identified on the POSS plates. This object showed a remarkable change in its appearance and the associated star from the original 1952 POSS plates. The discovery of this object was independently reported by Cohen, Kuhl and Harlan in 1977 and by Russian astronomers Gylul'budagyan, Magakyan and Amirkhanyan also in 1977. (Free gift if you can pronounce these names in less than 5 seconds).
The original POSS plate from 1952 showed a nebulous streak similar to a comet’s tail about 1 arc minute long some 30" from the faint red star PV Cephei. See Fig. 1. The 1977 plate showed the nebulous streak had disappeared, and a fan shaped nebulous patch appeared with the considerably brightened star at its apex. Cohen quickly examined five available plates from the Lick Observatory archive covering the time period 1951 - 1975. The older plate showed the streak, but not the star, the newer plates showed the star but the streak was missing. Cohen's magnitude estimates of the star were:
POSS 1952... 18.5
August 1975.. 18.5
December 1976.. 16.9
July 1977.. 15.5
Why did the nebulous streak disappear ? And why did the fan-shaped nebula suddenly appear? Cohen has suggested that as the star brightened by some 3 magnitudes, it has begun to illuminate the fan-shaped nebula while the streak may have been a temporary phenomena such as the remnant of a massive stellar flare. This object resembles FU Orionis, that is a young star just beginning its hydrogen burning (main-sequence) life. Recall FU Orionis rose from magnitude 16 to 10 in 1936 and still remains at maximum. Cohen had originally suggested in his 1977 paper that the fan shaped nebula is a reflection nebula illuminated by the rapidly brightening star PV Cephei. Cohen remarked that in order for the streak to disappear on a time scale of 20 years it would require a very high ejection velocity of some 5,000 km/sec. As the material from the streak was ejected outward, it dispersed and its reflected brightness simultaneously decreased. At the same time as the star PV Cephei was brightening, this provided the light to illuminate the newly appeared fan shaped nebula.
In the 1980's Russell Levreault of the University of Texas made detailed observations of this unusual object at radio wavelengths. His model suggested an active pre-main sequence star with a strong stellar wind which is surrounded by a circumstellar disk. The disk acts to channel the stellar wind into two opposite directed streams causing the observed bi-polar flow. Levreault's observations showed the fan shaped nebula was causing blue-shifted flows in the direction of which the fan opens. The redshifted flows appear to be constrained by higher densities of the surrounding interstellar medium. The visible fan-shaped nebula according to Levreault is not ejected material both rather the result of a tunnel or cavity carved out by strong stellar winds of the surrounding molecular cloud material. Starlight escaping into its cavity reflected off its walls produces the visible nebula.
The visible flows have a size of about 1.5 parsec, mass of about 0.8-2.6 M¤ and an apparent age of 100,000 years. The optical appearance suggests that the disk powering the bipolar flows has an inner diameter of 0.01 parsec (2000 AU’s)
Theoretical and observational insights seem to imply a prevalent scenario for protostars and proto planetary systems. These studies are made in the infrared and radio wavelengths, as it is the only light that can emerge from the dense opaque regions. When a star has begun its hydrogen burning phase and has “lit up”, several things occur: formation of an accretion disk, violent gas jets, bi polar flows, and strong stellar activity. The most famous recent example is the highly studied dust disk by both ground based and the Hubble Space Telescope around Beta Pictoris in the southern constellation of Pictor. PV Cephei is circumpolar for observers in the United States and is visible near the meridian in November evenings.
The only image I have seen of this object this decade is in John Vickers CCD Atlas (page 227, image taken 1994). The nebula still appears fan shaped with PV Cephei near its maximum brightness. Any CCD imagers ready to see what the nebula looks like in 2004?
Figure 1. PV Cephei and the Disappearing Nebula. Left image is from the POSS plates 1952. Notice the nebulous streak resembling a comet’s tail. Right image taken by Martin Cohen in 1977. The streak is gone, replaced by a fan shaped nebula with PV Cephei at the apex. Is this a new star and solar system in the making?
REFERENCES
Cohen, M., Kuhi, L., Harlan, E., 1977, A Remarkable Structural Change in a Faint Cometary Nebula, Astrophysical Journal, 215, L127-L129.
Cohen, M., 1980, Red and Nebulous Objects in Dark Clouds: A Survey, Astronomical Journal, 85, p. 29-35.
Levreault, R., 1984, Interactions Between Pre-Main-Sequence Objects and Molecular Clouds. II. PV Cephei, Astrophysical Journal, 277, p. 634-639.