Alert Notice 686: B-band photometry of TYC 2597-735-1 urgently needed

November 19, 2019

AAVSO Forum thread (scroll to the bottom of a thread for latest posts):
- Campaigns and Observation Reports: https://www.aavso.org/tyc-2597-735-1-campaign-2019

Dr. Keri Hoadley (California Institute of Technology) requests AAVSO observers' assistance in obtaining B-band images of the star TYC 2597-735-1 over the next few weeks. This campaign needs to be accomplished before the target becomes unobservable in the West.

Dr. Hoadley writes: "This is both a long term study and follow-up to a previous campaign spanning over a century. We have a suspicion that TYC 2597-735-1 is secularly dimming very slowly, as well as an idea of what is causing this (which we cannot enclose here, as these images and all other results of our studies will be submitted to Nature for publication). We cannot complete our analysis of this dimming without more modern B-filter photometry, since the last data we have are from 1989 and the data sets we have show two possible light curve dimming solutions, and we need modern B-filter photometry to distinguish which scenario is correct.

"We need roughly 10 nights' worth of images, with 10 images taken per night using a CCD with a [Johnson B or Bessel] B filter; the nights do not have to be consecutive but should not be spaced out over [more than the next few weeks]."

DSLR and visual observations are not requested "...because we will require as close to Johnson B filter images as possible, to match the photometry in our long-term light curve." Spectroscopy is also not requested.

"The individual series of 10 images in a fast row is to beat down the measurement errors, as well as to provide a measure of the internal errors (from the RMS).  We want ~10 of these individual runs, by some combination of multiple observers and multiple nights...useful data sets would be five individual observers each taking anseries of 10 fast images on two nights.  Or it might be two observers taking 10-exposure-series on five nights each.

"For SNR: If the peak counts in the brightest pixel of the target star are around 10%-40% or so of the saturation or nonlinearity level of the CCD, then the electrons in the star image >>10,000, and the Poisson-noise-only error bars <<0.01 mag. In this case, the true total error bars will be dominated by the various systematic effects, and these are handled elsewise.  For 11th mag stars and the likely scope apertures, exposure times of ~60 seconds should be adequate.  So the whole series of 10 images per night goes fast.

"The CCD FoV should be sufficiently large to record all four comparison stars and the target, i.e., 0.6°x0.6° (36’X36’) or larger.  It is adequate to have a smaller CCD FoV of 0.3°X0.3° (18’X18’) to record only three of the comparison stars [see the table for which comparison stars have priority].

"We need all the processed images (taking into account darks/flats/biases/etc.) themselves in FITS format. Be sure to specify whether a Johnson or Bessel B filter was used. Please post images (copy/paste URL into browser if necessary) at: https://drive.google.com/drive/folders/11HVuVqUwLPk_P0EtRADkNLnBaN0_VnvY?usp=sharing

In addition, please provide the instrumental magnitudes of all stars in the table that appear in the images; email them to khoadley@caltech.edu.

"The target is in the Keystone of Hercules, and this is available only in the evening around astronomical twilight for the next few weeks. The stars are bright enough that working in late twilight is fine, but do not let the target get much below 25 degrees above the horizon or so. The night must be clear of clouds, but small wispy clouds or haze layers are OK."

Primary Target = TYC 2597-735-1
Comparison Stars = TYC 2597-1026-1, TYC 2597-458-1, TYC 2588-182-1, (TYC 2588-1116-1)
If possible, please include the first 3 comparison stars in the field of view of the target. If possible to fit all 4, that would be ideal but not necessary. We need as many comparison stars in the same image as the target as possible.

Finder charts identifying TYC 2597-735-1 may be created using the AAVSO Variable Star Plotter (VSP). The comparison stars listed in the above table are shown on the charts. Note that their magnitude values shown on the charts are not the same as provided by the PI in the above table (magnitudes shown on the charts were taken from APASS and appear in the APASS V-mag and Chart V-mag columns above).

If B magnitudes are obtained from the images for submission to the AAVSO International Database (not requested by the PI), please be sure to use the APASS magnitudes given in the VSP photometry table. Submit observations  to the AID using the name TYC 2597-735-1.

This Alert Notice was compiled by Elizabeth O. Waagen using information supplied by Dr. Keri Hoadley.

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