Alert Notice 851: Multiwavelength coverage requested for V695 Cyg study with HST

February 14, 2024

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Dr. Philip Bennett (Dalhousie University) has requested AAVSO observers' assistance in obtaining photometry (all modes and visual) and spectroscopy of the spectroscopic eclipsing binary red supergiant V695 Cyg (= 31 Cyg = HR 7735= HD 192577) in support of observations scheduled over the coming months with the Hubble Space Telescope.

This campaign begins immediately and will run through December 2024. CCD, PEP, DSLR, and visual photometry and spectroscopy are requested. The most useful data for observers to obtain are V, and especially U & B, photometry, and spectroscopy at wavelengths < 4500 A.

Dr. Bennett provides the following information:

V695 Cyg is a spectroscopic, eclipsing binary: spectral types K4 Ib + B3 V, with period 3784.34 days (=10.36 years). The K4 Ib red supergiant primary dominates the optical flux, but the hot B-type companion contributes significant flux shortward of 4500 A.

HST will observe the evolution of the ultraviolet spectrum at 10 epochs at wavelengths < 3100 A during the partial, total, and near-eclipse phases of 31 Cyg through September 2024.

The goal of this program is to determine the amount of circumstellar absorption from the wind of the red supergiant throughout the eclipse period. These observations will allow the density, velocity, ionization, and temperature structure of the red supergiant’s wind to be mapped, and an accurate mass loss rate determined for this important red supergiant.

The campaign begins immediately and continues through the end of May with higher cadence and through December 2024 with reduced cadence.

The start of the 2024 eclipse is imminent, with the following estimated timeline of events:
-    start of U-band partial eclipse (1st contact): 2024 Feb 17: JD 2460357.7
-    start of U-band totality (2nd contact): 2024 Feb 21:  JD 2460361.70
-    mid-eclipse on 2024 Mar 22: JD 2460391.72
-    end of U-band totality (3rd contact): 2024 Apr 18: JD 2460419.36
-    end of U-band partial eclipse (4th contact): 2024 Apr 23: JD 2460423.70

Observing cadence (photometry and spectroscopy)
 -  Nightly: Feb 10 – Feb 13 (immediately prior to ingress – 1st contact)
 -  Several times per night: Feb 14 – Feb 24 (partial ingress phases: 1st – 2nd contact)
 -  Nightly: Feb 25 – Apr 14 (total eclipse)
 -  Several times per night: Apr 15 – Apr 26 (partial egress phases: 3rd – 4th contact)
 -  Nightly: Apr 27 – May 31 (immediately following egress – 4th contact)
 -  Weekly: June 1 – Dec 31

Time series of Johnson B and U via CCD and PEP are most useful. Please alternate filters if both U and B are available. V observations are also requested, as are DSLR and visual observations.

 - Wavelength range of interest in Angstroms: 3200 – 4500 A
 - If full coverage isn’t possible, specific lines which should be prioritized: the region of the Ca II H & K lines (3900 – 4000 A)
 - Resolution: higher spectral resolution preferred, but any spectroscopic data will be useful
 - Signal to noise ratio: S/N > 30 preferred, but lower quality data are still useful
 - Cadence required for spectroscopy: the same cadence as for photometry (see above)

Observers with spectroscopic data should contact Philip Bennett at <> as well as send spectroscopy reports to the AAVSO Spectroscopy Database (AVSpec, link below).

Coordinates (2000.0):  R.A. 20 13 37.91  Dec. +46 44 28.8    (from VSX entry for V695 Cyg)

Range:  3.73 - 3.89 V  (from VSX)
Brightness information provided by Dr. Bennett:
     V=3.80, B= 5.08, U= 5.50 (out of eclipse)
     V=3.9,   B= 5.6,   U= 7.1    (total eclipse)

Charts: Charts with comparison stars for V695 Cyg may be created using the AAVSO Variable Star Plotter (VSP).

Submit observations: Please submit observations using the name V695 CYG.
  - Submit optical observations to the AAVSO International Database using WebObs
  - Submit spectra to the AAVSO Spectroscopy Database (AVSpec) and contact Dr. Bennett.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material supplied by Dr. Bennett.

Information on submitting observations to the AAVSO may be found at:
 - Photometry/visual observations:
 - Spectroscopy:


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