Decline In Brightness Research Articles

Nova in Cygnus of record brightness

THE recent bright nova in Cygnus is probably one of the most remarkable galactic novae observed. Invisible on the Palomar Sky Survey it had climbed through at least 18 mag to its maximum of V = 1.8 on August 31.0 (UT). This led to some early speculations whether this might be the first galactic supernova observed since Kepler's days. This suggestion is not, however, supported by the spectrum of the object and is also inconsistent with the extremely rapid decline in brightness after maximum. On the other hand, assuming it to be a classical nova, there are indications that it is a very luminous one.

A Spectroscopic and Photometric Study of the Pulsating R Coronae Borealis Type Variable, RY Sagittarii

Results are given of an intensive study of RY Sgr during the period 1967–70. The rapid decline in brightness at the beginning of this period and the gradual rise to maximum light are covered. 382 emission lines (mostly identified) were found during the initial decline as was a continuum attributed to the electron attachment spectrum of CN. The development with time of this emission line chromospheric spectrum indicates both a lowering of the excitation and a decrease in the effects of self-absorption. The chromosphere decays on a time scale of ~ 22 days. The initial decrease in photospheric radiation has a time scale of ~ 5 days or less. The absence of [Fe II ] emission suggests that the density in the emitting region is greater than ~ 10 14 atoms cm –3 in the early stages of the decline if collisional excitation is dominant. During the decline displaced shell lines of Ca II ( – 200 km s –1 ) were present. On the rise to light maximum and at maximum, the light and colour curves show smooth variations |$(\Delta V\,\sim\,{0}^\text{m}\cdot5\,\Delta (B-V)\,\sim\,{0}^\text{m}\cdot3\,\Delta (U-B)\,\sim\,{0}^\text{m}\cdot5)$| with a period of 38·6 days. A similar periodicity is present in the radial velocities (amplitude |$\sim\,30\,\text{km}\,\text{s}^{-1}$| and it is concluded that RY Sgr is a pulsating variable (as originally suggested by Jacchia) as well as an RCB star. At some epochs on the rising branch the spectrum is normal (i.e. as at maximum) but the star is faint and red. This together with the infra-red observations of Lee and Feast strongly supports the hypothesis of extinction by a circumstellar dust shell. Values of |$A\text{v}/{E}_{B-V}\,=\,4\cdot3\,\text{and}\,{E}_{U-B}/{E}_{B-V}\,=\,1\cdot3$| are found for the shell. Abnormal spectra and colours follow a drop in light on the rising branch. They are apparently due to the filling in of the photospheric absorption lines by chromospheric emission. The primary cause of this phase seems to be a decrease in photospheric light, possibly due to the formation of a fresh dust shell near the photosphere. Displaced shell absorption lines (Ca II and Na I ) are present at this stage. The pulsations of RY Sgr are broadly consistent with recent work by Trimble on the pulsation of helium stars.

Yearly Variations of 3C 120

THE photographic light curve1 of the Seyfert galaxy 3C 120 shows a large-amplitude (∼1.2 magnitudes) long-term decline in brightness over five consecutive seasons from 1934/35 to 1939/40. Short-term fluctuations of smaller amplitude (∼0.3 to 0.6 magnitudes) with decline times of about 200 days are apparently superimposed on this decline1, and the question immediately arises whether these fluctuations are intrinsic to the source or are due to some yearly observational selection effect. Kinman2 has pointed out the existence of an hour-angle effect for 3C 345 and inspection of the hour angles of the 3C 120 plates reveals that the hour angles are selectively larger at the beginning and end of the season than in the middle. In this report we conclude that the yearly fluctuations in 3C 120 are almost certainly intrinsic to the source.

Spectroscopic and Photometric Observations of S Doradus

The P Cygni character of the spectrum of S Dor has been confirmed. The radial velocity does not vary during the three years covered by the observations. The mean emission-line velocity is +295 km/sec and therefore the membership of the Large Magellanic Cloud is beyond question. The mean displacement of the absorption lines is 82 km/sec towards the violet; this is unusually small for a P Cygni star. The presence of helium is doubtful. The spectral type is A0. The Ca II lines are mainly stellar. Lines due to H and Can are strong. For 74 lines wave-lengths and intensities are presented together with probable identifications. Photoelectric observations in an interval of seventeen months show a decline in brightness of 0m.8 without change in colour. During this same interval the emission character of the spectrum became more pronounced, while forbidden lines of Fe II, which are diffuse, strengthened with respect to the permitted lines. In 1955 February the apparent photographic magnitude was 10m.8; the corresponding absolute magnitude is −9m.2. S Doris not an eclipsing variable but varies irregularly.

Astronomical Topics

The Delporte Planet.—A Science Service Bulletin of April 18 describes the views of Drs. F. L. Whipple and L. E. Cunningham on this object. They have obtained a value of the period as approximately 3 years, which does not differ greatly from Dr. Kahrstedt's value of 2¾: years. But they prefer to call it a comet, not a minor planet, on the ground of its rapid decline in brightness, which certainly was much greater than would be expected in a planet. They still suggest that the body is identical with the comet Tuttle-Giacobini, seen in 1858 and 1907. The writer of this note held a similar view when the body was first discovered, but has abandoned it, on the ground that the observations of the comet of 1858 will not admit of a period much less than six years, while the present period is less than three years. Jupiter is the only planet that could produce such a great change of period, but if Jupiter had been the cause of the change, the orbit would still pass close to Jupiter's orbit, which it does not. The least distance between the orbits is about two units.