Observing the 2017–’19 primary eclipse of VV Cephei with a low-resolution spectroscope

2021 July 31

Hugh Allen

Introduction

VV Cephei is a variable star that shines brightly at around 5th magnitude, even though it is 4,900 light-years away.¹ Its spectrum was first described in 1912 by Annie Jump Cannon when she was working on photographic plates at the Harvard College Observatory.² In 1937 she communicated to Sergei Gaposchkin a review of all the spectra of VV Cep in the Harvard collection:

The spectrum…appears on twenty-one photographs taken…between the years 1887 and 1927. The star…was classified as Map, the peculiarities being as follows: bright hydrogen lines form a series decreasing in intensity from Hβ to Hζ; the spectrum extends into the violet as far as spectra of Class B; the wide absorption bands, H and K, are very hazy and indistinct. The exceptional appearance of the spectrum occurs on a photograph taken September 24, 1895…The spectrum was then of a normal Class M…The violet region is not seen of shorter wavelength than H and K, which are strong and distinct. The hydrogen lines are not bright.³

A similar contrast is seen in a pair of images from this project (Figure 1).

After the suggestion by Dean McLaughlin in 1936 that VV Cep was an eclipsing binary,⁴ Gaposchkin (1937) made a detailed study of the available photometric and spectroscopic observations covering 1890–1937.³ The magnitudes measured from blue-sensitive photographic plates gave a light curve that displayed three dips of about 490 days duration: 1895–’96, 1915–’17 and 1936–’37, with a resulting period of 7,430 days (20.3 years). By combining this with measurements of Doppler shifts in the spectral lines, Gaposchkin proposed that the VV Cep system consisted of a 44-solar-mass (M_☉) cool, red supergiant and a 35M_☉ hot blue star – a rare example of a red supergiant binary. The hydrogen emission lines were associated with the hot star and so disappeared during its eclipse. The plane of the stars’ orbit lay almost along our line of sight. The supergiant’s radius was estimated as 2,400 solar radii (R_☉), a hundred times greater than its hot companion, explaining the brightness of such a distant star.

Four more eclipses have now passed and VV Cep has featured in nearly 160 refereed papers. Measuring the parameters of the system has not been easy. Pulsations and other effects produce intrinsic brightness variations in the red supergiant, with several overlapping periods,⁵ and due to its huge size the supergiant has a semi-transparent outer atmosphere so that the eclipses begin and end only gradually.⁶ The size of the supergiant has been variously estimated between 600 and 1,800R_☉, whilst the mass estimate is now nearer 20M_☉ for both stars.⁷ Photometric estimates of the full duration of eclipses have varied between 490 and 635 days, and the duration of totality between 450 and 498 days. Successive measurements of the eclipse midpoints showed that at the last 1997–’98 eclipse, the midpoint occurred approximately 60 days later than expected (Table 1).

In 1943 Victor Goedicke published the first comprehensive investigation of the spectrum of VV Cep. He noted how ‘upon eclipse, the early type [hot blue star] spectrum disappears, after which the hydrogen emission lines weaken and disappear, in order of increasing wavelength…Upon emergence from eclipse the procedure is reversed’. He also observed that the intensity of the hydrogen emission lines was ‘variable in an irregular manner’.⁶

The disappearance of hydrogen emission lines from H-beta (Hβ) downwards during eclipse indicates that these lines arise principally near the hot blue star, from an envelope of gas which is accreted from the red supergiant’s stellar wind. The H-alpha (Hα) emission line never completely disappears during an eclipse, indicating that the Hα emission region is more extensive. In medium- and high-resolution spectra the brightest lines, Hα and Hβ, show blue- and red-shifted wings separated by an absorption trough, suggesting a circulating disc structure around the hot star’s equator.¹⁰ During the most recent eclipse, 2017–’19, a high-cadence campaign of medium-resolution spectroscopic observations by amateurs has potentially uncovered a 43.5-day periodicity of the variations in the Hα emission.¹³ The classification of the hot blue star is still uncertain, ranging from late O- to early A-type.¹⁴

Since 2015, an international group of amateurs has informally cooperated using low-resolution spectroscopy (resolving power R = 500–1,000 at Hα wavelength) to observe the 2017–’19 primary eclipse of VV Cep. By maintaining regular observations, it has been possible to show for the first time how low-resolution spectra obtained by amateur astronomers can contribute to our understanding of this rare and enigmatic star.

Observations

There are an increasing number of moderate-cost, low-resolution spectroscopes in the hands of amateurs, such as Shelyak Instruments’ Alpy 600.¹⁵ The approaching VV Cep eclipse was well publicised and the first contact of the circulating disc of gas was anticipated in mid-2017 (Figure 2).

Together with the author’s own observations, low-resolution spectra of VV Cep have been collected since 2015 July either by direct submission to the author or by downloading from the public BAA and ARAS Spectroscopy Databases.¹⁶ Details are given in Table 2. By pooling data in this way, a high observing frequency was achieved, especially during some of the critical phases of the eclipse. Signal-to-noise ratios in the range 125–350 were typically obtained, adequate for reliable measurement of features in the spectra. Using free software such as ISIS or BASS, spectra were wavelength calibrated, and the continuum shape corrected for instrument and atmospheric effects using a reference star at a similar altitude in the sky. The author used the reference star nu Cep (SAO 19624), comparing with a non-interstellar-extinction-corrected version of the nu Cep spectrum from the Miles database (file S815). A typical example of the final appearance of the reduced VV Cep and nu Cep spectra is shown in Figure 3.

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