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Alert Notice 554: Observations of V420 Aur (HD 34921) needed to support spectroscopy

Note added October 4, 2016: In the original version of this Alert Notice, V420 Aur was referred to as V420 Oph in the sections on creating charts and reporting observations. Also, the range was given as V=7.15-7.35  when the correct value is V=7.35-7.51. The text below has been corrected. Apologies to the PI's for the error.

October 3, 2016: Marcella Wijngaarden and Kelly Gourdji (graduate students at the University of Amsterdam/Anton Pannekoek Institute for Astronomy) have requested AAVSO observers' assistance in providing optical photometry in support of their high-resolution spectroscopy with the Mercator telescope + Hermes spectrograph in La Palma 2016 October 7 through 17.

They write: "[V420 Aur (HD 34921) is] the optical Be star that is part of a peculiar High Mass X-ray Binary...[that exhibits highly] complex and variable is difficult to construct a physical model of this HMXB system. Though based on these observations, the system is thought to contain a B[e] star with a dense plasma region, an accretion disk around a neutron star, a shell and circumstellar regions of cold dust. It has been over a decade since the last spectra were taken, and, given the highly variable nature of this star, we expect new observations to yield new information that will contribute to a better understanding of this system."

Observations in BVRI are requested beginning immediately and continuing through October 24. The PI's write: "Observations using other filters will be useful as well as long as there are multiple observations in these bands. In all cases, timeseries in a few bands (i.e. BVRI) are preferred over single/a few observations in the other bands as it is the variability on relatively short timescales that is most important. Unfiltered observations are in this case also useful but preferably only when they are available for multiple nights (in order to cross-match with other observations).

"The target is bright so exposures should be long enough to reach good signal to noise in order to see the small variability amplitude but without saturating the target/comparison stars." With a range of V = 7.35 - 7.51, V420 Aur is a good candidate for PEP, DSLR, and CCD photometry. Its very small amplitude makes it a challenging target for visual observations.

The PI's continue: "We will study the variability on several timescales, so observations starting from a few per night to high cadence timeseries are useful. Especially in the [October 7-17] period the higher rate the better.

"Observations starting as soon as possible are ideal, for these the requirements are the same as above...We request the same observations until one week after the spectroscopic observing run...[so] that hopefully we have enough to phase fold the data."

Coordinates (2000.0): R.A. 05 22 35.23  Dec. +37 40 33.6

Charts with a comparison star sequence for V420 Aur may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations to the AAVSO International Database using the name V420 AUR.

This campaign is being followed on the AAVSO Observing Campaigns webpage. A thread has been created on the Campaigns and Observation Reports forum at (

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.


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