AAVSO: AAVSO Infrared Photometry Progress Report: A Highlight on Eta Aql August 2005

The infrared photometry group of the AAVSO has been busy the last couple of years taking observations with their SSP-4 photometers and developing the procedures necessary to have a successful observing program. Between May 2003 and April 2005, the group has reported over 600 J and H band observations to the AAVSO. The active observers in the group are: Dirk Terrell, Boulder, Colorado; Jim Wood, Bakersfield, California; W. Doug Hodgson, Chidlow, Australia; Ken Luedeke, Albuquerque, New Mexico; and Doug West, Mulvane, Kansas. Two other AAVSO members have SSP-4 photometers and may start taking observations soon.

The SSP-4 photometer is different from the SSP-3 in that the detector is thermoelectrically cooled to -40 C. The cooling cuts down on the thermal noise and gives a higher signal-to-noise ratio. Even with the cooling, the InGaAs detector in the SSP-4 isn't nearly as sensitive as the Silicon detector in the SSP-3. As a result of this lower sensitivity the SSP-4 with a 10" scope is limited to about 4th magnitude. This may sound like a really big limitation; however, there are still hundreds of objects to observe in the J (1.25 micron) and H (1.65 micron) bands. A good instruction to the SSP-4 and infrared photometry can be found in the West 2004 article.

One of the stars receiving a lot of attention from the group right now is Eta Aql (AAVSO 1947+00). Eta Aql is a classical Cepheid variable of the delta Cephei type. Visually, this star varies from approximately magnitude 3.5 to 4.6. The comparison star used for the infrared photometry is SAO 125235 (J=2.19 and H=1.72). This report concentrates on the J and H band observations.

The visual observations used in figure 1 are from AAVSO visual observations taken from the Quick Look file over July and August of 2005. The J and H band observations were gathered by the AAVSO IR Photometry group and were taken from April 2004 until August 2005. The phase diagram is reduced using a period of 7.1766 days and an epoch (JD) of 2432926.749 (Kukarkin). Note the phase difference between the visual and infrared observations.

Figure 2 — Phase plot

Figure 2 is an expanded view of the infrared observations and includes error bars. The error bars represent the standard deviation of the measurements. The fainter observations (in black) with the larger error bars are J band. The brighter observations (in purple) with the smaller error bars are H band. The larger error bars in the J band observations are due to the fainter signal and thus a lower signal-to-noise ratio. The J band observations vary by approximately 0.5 magnitude and the H band observations vary by 0.3 magnitude.

Figure 3 — Averaged Phase Diagram for Eta Aql.

To reduce the noise in figures 1 and 2, the observations are averaged in figure 3. To create each of the data points on the graph, the phases associated with two observations are averaged and the magnitudes are averaged using the standard deviation of the observations as weighting factors. The average phase represents the mid-point of two phase points; this is a mid-point running average. To cover the end points, that is, phase of zero and one, points are "wrapped" from higher and lower phase values to generate these two points. Using the standard deviation (error bars) as weights makes the curve more representative of the actual phase curve for the star.

So far, my literature search has revealed three papers, Welch, et. al., Hindley and Bell, and Barns III et. al. that have published J and H band photometry for eta Aql. Their phase curves are similar to what is presented here. There is one striking difference; the AAVSO phase curve has over 50 percent more observations than any of the previous papers. Once the AAVSO phase curve is published in the JAAVSO, it should be the definitive phase plot for this variable star. More observations are planned before eta Aql sets in the West to end this observation season. This will add more detail to the phase plot.

Eta Aql is just one of the stars being monitored in the AAVSO IR Photometry Program. Thanks to all the SSP-4 observers for their hard work and new SSP-4 observers are always welcome.

References:

Welch, D.L., Wieland, F., McAlary, C.W., McGonegal, R., Madore, B.F., McLaren, R.A., and Neugebauer, G., "JHK Observations of Classical Cepheids", The Astrophysical Journal Supplement Series, 54:547-579, 1984 April.
Hindsley, R.B., and Bell, R.A., "The Cepheid Infrared Period-Luminosity Relations", The Atrophysical Journal, 348: 673-681, 1990 January 10.
Barnes III, T.G., Fernley, J.A., Frueh, M.L., Navas, J.G., Moffett, T.J., and Skillen, I., "BVRIJHK Photometry of Cepheid Variables", Publications of the Astronomical Society of the Pcific, 109: 645-658, 1997 June.
West, D., Single Channel Infrared Photometry with a Small Telescope, WestSkies Observatory, Mulvane, Kansas, USA. Invited talk at the 1st Virtual Meeting on Amateur Astronomy. 2004.
Kukarkin, B.V., Kholopov, P.N., Pskovsky, Y.P., Efremov, Y.N., Kukarkina, N.P., Kurochkin, N.E., and Medvedeva, G.I.; "General Catalogue of Variable Stars", 3rd ed., 1971.