V442 And biography

Today the observatory had a new addition: a new 18V power supply for the Titan mount. I also got a spring load (electronic) so I could adjust wih more care the balance of the mount. I added 100gr on the counter weight arm as telescope is pointing East (my usual observing side).


While observing several Be stars including V442 And, I am reading several article on V442 And (HD6226) for which I discovered an outburst on august 21st and whose emission profile has been changing every day since then...

In 1967, Bertaud and Floquet wrote a spectroscopical observation report on several peculiar A-type star with metallic lines. Based on 29 january 1960 observation, they indicate that the K line is thin and intense; all lines give a B8I spectral type. HD6226 can't be considered as a peculiar A type star.

In 1988 Bidelman studied several stars with variable radial velocities. HD6226 is then classified as a B3III star.

In 1998 Bozic & Harmanec published an article specifically on HD6226. Based on photometry (including Hipparcos data), they suspected HD6226 to be a Be star with similar behaviour as QR Vul (another Be star for which we have found several outbursts during last ten years).
They found a potential period of 481.3 days for HD6226.

In 2000, McCollum et al. reported the first detection of Halpha emission line. Another report from 2003 indicated that the emission was not seen on spectra taken during November and December 2002 but reappeared on spectra taken between January 19 and March 15, 2003.

In 2004, Bozic & Harmanec published again a dedicated (and long) article on HD6226 (Christian Buil being a co-author!). They found a stellar rotation period of 2.61507 days on He 6678 He I and other metallic lines. They also suspected a period of 24-29 days and a long cycle of 630 days with different durations of the individual cycles for strong emission line episodes.
Their physical parameters for HD6226 are: Teff of 17000K, log g of 3.0 [cgs], 5 solar masses, 11 solar radii. The equatorial velocity is estimated at 213 km s-1 (very close to the break-up limit of the Roche model: 240 km s-1) and the inclination at 19° (almost pole-on).
For them, HD 6226 is probably one of the first B stars for which the Be nature was predicted on the basis of the character of its light and colour changes.
They also mentionned that variability on a time scale of hours cannot be excluded completely – especially since the position of the star in the HR diagram is at the edge of the β Cep instability strip. They couldn't find evidence in photometry for variation in less than 0.4 days but some spectra provide some evidence of moving sub-features travelling accross the He I 6678 line profile. They conclude that HD6226 is a line-profile variable.
They noted an important information on the correlation between photometry (variability in the 0.2 mag range) and spectroscopy (peak intensity of Halpha (V+R)/2): "We note that the observed sequence of events is typical of the positive correlation between brightness and emission strength, as defined by Harmanec (1983, 2000) and discussed semi-quantitatively for another Be star, V839 Her =4 Her by Koubsk´y et al. (1997). Our interpretation is the following: The initial formation of the envelope manifests itself as a pseudophotosphere, a region above the stellar photosphere which is optically thick in the continuum. Since we probably observe HD 6226 more pole-on than equator-on (considering its low v sin i – see below), this pseudophotosphere acts to increase the observed radius of the star which naturally leads to brightening of the object and its apparent evolution from the main sequence towards the supergiant sequence in the colour-colour diagram. As the envelope grows, it gradually gets optically thin in the continuum but opaque in the Balmer lines and this leads to the development of Balmer emission lines and a gradual decrease of the brightness of the object to its undisturbed level."
Regarding the double peak, they mentionned: "It is clear that the inclination under which we observe the star must be low. This, of course, raises the question of the presence of a sharp central absorption seen during the emission episode in the Hα emission line. One possible interpretation is to assume that the Be envelope is sufficiently spheroidal to produce absorption effects even above the poles of the star. Another one would be to assume that the narrow absorption comes from a secondary in a putative binary system. It seems clear, however, that it cannot originate in a stellar wind from polar regions of the star since its RV blueshift with respect to the stellar photosphere – if any – is smaller than 5 km s−1."
Their concluded with: "Clearly, HD 6226 is a very interesting Be star which deserves
further intensive study."

In 2013, Sigut & Patel summarized the correlation between photometry and spectroscopy and proposed a model.
"A positive correlation is seen for most Be stars for which the relevant observations are available, and it is characterized by a decrease in the (visual) magnitude of the system with increasing strength of the Balmer Hα emission (such as an increase in its equivalent width in emission).
In a (U − B)
versus (B − V ) color–color diagram, Be stars with a positive correlation change their luminosity class but not their spectral type (Harmanec 1983). Increasing Hα signifies an increasing disk, and it therefore seems natural to expect an increase in the overall brightness of the system, star-plus-disk, as the disk is built."
V442 And fit into this category of positive correlation, with a low inclination (i=19°).
"On the other hand, an inverse correlation, as described by Harmanec (1983), is characterized by an increase in the magnitude of the system with an increase in the strength of the Balmer emission. In the (U − B) versus (B − V ) color–color diagram, Be stars that show an inverse correlation move along the main sequence, changing their spectral type, but not their luminosity class (Harmanec 1983).
Fewer stars are known to exhibit this inverse correlation: 4 Her (B9e; Koubsky et al. 1997), 88 Her (B7pshe; Doazan et al. 1982), V1294 Aql (B0Ve; Horn et al. 1982), and η Cen (B1.5IVne; ˇStefl et al. 1995).
An inverse correlation is thought to occur when a Be star disk is viewed more edge-on (i.e., at higher inclination angle i; Harmanec 1983). The forming disk can then act to reduce the brightness of the system by blocking the light from the stellar disk, while the small projected area of the disk on the sky keeps the disk emission to a minimum. If the critical inclination angle required to observe an inverse correlation is large enough, then inverse correlations will be statistically less likely to be found than positive correlations."


References
Bertaud C, Floquet M; "Observations spectrographiques d'etoiles A a spectre particulier et a raies metalliques"; Journal des Observateurs, Vol. 50, p. 425 (1967)

Bidelman W; "spectral types of 80 early-type stars of variable radial velocity"; PASP 100, p828 (1988)

Bozic H, Harmanec P; "HD6226: a new bright B variable with occasional brightenings. Is it an unrecognized Be star?" A&A 330, p222 (1998)

Bozic H, Harmanec P, et al.; "Properties and nature of Be stars , XXII. Long-term light and spectral variations of the new bright Be star HD 6226"; A&A 416, p669 (2004)

McCollum, B. et al.; "First Detection of Hα Emission in the Bright, Variable B Star HD 6226"; American Astronomical Society, 197th AAS Meeting (2000)

Sigut T., Patel P.; "the correlation between Halpha emission and visual magnitude during long term variations in classical Be stars"; ApJ 765 p41 (2013)