R CrA & cyclic brightness variations in NGC 6729

2022 January 29

Terry Evans & Grant Privett

Introduction

‘Variable nebulae’ represent a small class of objects whose appearance changes significantly over timescales of years, months, or even days. They have been known since the 1800s,¹ with the best known being Hubble’s Variable Nebula (NGC 2261) in Monoceros, Hind’s Nebula (NGC 1555) in Taurus and NGC 6729 in the southern-hemisphere constellation of Corona Australis. In recent years the list has been added to, most notably with McNeil’s Nebula (2004), and Borisov’s Nebula (2020).

Since the introduction of affordable low-light sensors during the 1990s, the observation of variable nebulae has become increasingly popular,¹ and in recent years amateurs have begun to obtain useful quantitative results for some nebulae.^2,3

NGC 6729 was discovered in 1861 by Julius Schmidt while using the 6.2-inch Plössl refractor at Athens Observatory.⁴ It was then independently found in 1864 June by Albert Marth, working with William Lassell on Malta.⁵ Marth noted it as a nebulous tail from a 13th-magnitude star, lying close to the nebulae now known as NGC 6726 and NGC 6727. Schmidt noted that the nebula and star were both variable, with the star varying over a ‘short’ timescale. This variability was later confirmed photographically with images captured from Hêlwan Observatory between 1911 and 1913 by Harold Knox-Shaw,⁶ who followed it for some years and eventually surmised that there was a relationship between the stellar magnitude and the appearance of the nebula.^7,8

NGC 6729 lies at the edge of a large, dark molecular cloud in which young stars are forming and illuminating the NGC 6726/7 nebulae. The molecular cloud extends over several degrees, but NGC 6729 is a small fan-like nebula roughly one-arcminute long, illuminated by the star R Corona Australis (R CrA) at its point and T CrA near the end of the fan. NGC 6729 appears to be a mixture of both reflection and emission components. Within the cloud and surrounding NGC 6729 lies a group of infrared sources known as the Coronet Cluster,⁹ of which T and R CrA are the most conspicuous members.

The distance to the Coronet Cluster of young stars was found to be 150 ± 4 parsecs (pc),¹⁰ according to a study of potential Gould Belt regions. That value is very similar to the first estimated figure of 150 ±50pc for NGC 6729, determined by Sergei Gaposchkin in 1936.¹¹ However, the star R CrA itself is, according to the Gaia DR2 catalogue, 96pc distant.¹² This anomaly appears to be caused by R CrA being a triple star, with two closely-spaced stars of 3.0 and 2.3 solar masses at a 0.2-arcsecond separation and an M-type low-mass dwarf in a 45-year orbit.¹³ The renormalised unit weight error assigned by the Gaia catalogue to R CrA (Gaia DR2 6731199293213061632) is 19.7, which indicates a poor fit to the single-star model used in that release,^12,14 thereby justifying considerable caution.

The comma-shaped Herbig–Haro object HH 100,^15,16 100 arcseconds to the south-west of R CrA, is seen against a dark nebula that is believed to attenuate the brightness of stars within it by up to 50 magnitudes.⁹ While HH 100 has been reported to be variable in the visible band, it appears to be associated with an infrared source only weakly detected in visible-band imaging using large instruments.^15,17

R CrA is a young star of approximately 1.5Myr age,¹³ which has been variously spectrally categorised, but is often reported as B5IIIe. The previously observed variability over a 65.767-day period is now attributed to the binary nature of the star.^13,18

NGC 6729 remains a target of active research within the professional community.

The data set

Images of R CrA and its environs were collected by Terry Evans during the years 2016 to 2020.

The images for 2018–’20 were acquired using a TMB 203 apochromatic refractor with an air-spaced triplet. The optical tube assembly was mounted on a Software Bisque Paramount ME4000, and the images collected via a Starlight Xpress Trius-694 CCD camera. Focusing was achieved using an Optec TCFS-i focuser and guiding via a Starlight Xpress Lodestar and a Pentax 75mm SDHF guide-scope. A TruTech filter wheel with an Astronomik L filter was employed. System control was via MaxIm DL. The system was hosted at the Riverland Dingo Observatory in South Australia.
Images were initially collected in batches of exposures, each of 300s duration, with dithering occurring between each frame and refocusing after every third frame. Flats, flat darks and darks were obtained for every data set and applied using Maxim DL. All frames were dithered to minimise cosmic ray and pixel defects when stacked.

Images were captured with NGC 6729 at altitudes of between 30 and 60 degrees. In practice, the elevation appeared to make little difference to the amount of detail apparent in the images, with nights of poor transparency being the biggest cause of discarded images. Of the 180 nights on which data were collected, images from 132 have been used in this study.

All data were saved as files in FITS format. Blind astrometric reduction was undertaken via the Astrometry.net website,¹⁹ to ensure the results were of consistent accuracy and that the world coordinate system (WCS) components inserted into the FITS files were compatible with exploitation via the Python AstroPy software library.²⁰ To ensure the best possible accuracy from Astrometry.net, the ‘Advanced’ control parameter – ‘Tweak’, employed to define the order of polynomial used within the WCS – was increased from its default value of two, to three.

Data processing

In the past, the extraction of photometric data from photographic plates of NGC 6729 was a slow and laborious process. While good results could be achieved, the effort was rarely expended and qualitative analysis of the nebula was more common – photometric measurement being largely reserved for R CrA itself. The advent of CCDs, combined with the ready availability of sophisticated image processing software and the high-level programming language Python 3, has made the collection and processing of data relatively simple and should, eventually, lead to an increasing number of detailed amateur reports regarding variable nebulae.

While all the data collection in this study employed only a luminance filter, and was thus not ideal for absolute photometry, previous work had suggested that worthwhile quantitative results could still be derived.² As the data set was created using a single telescope and CCD combination, it was hoped that the results would be more consistent than those seen previously,² as observer-to-observer offsets were eliminated.

Photometric solutions were derived for each image using the Gaia DR2 data set,¹² as well as bespoke Python software. Care was taken to ensure that saturated or nearly saturated stars were not employed in the reductions, with any star containing a pixel count above 55,000 discarded. Similarly, the 5% of stars with most extreme colour indices (Gaia bp-g) were discarded from the analysis.

To test the reproducibility of the approach, aperture photometry was obtained of three comparison stars lying close to R CrA. These stars are not reported to be variable.

The slightly larger scatter in the 2020 values arises from the increased use of 60-second exposures, rather than those of 300 seconds. Measurement of all three stars showed a variance smaller than 0.1 magnitudes. The stars were measured as Gaia g magnitude 15.02 ± 0.06, 13.34 ± 0.07 and 14.34 ± 0.05. Their Gaia Catalogue values are 14.46, 13.06 and 13.80 respectively – suggesting, perhaps, that they are significantly reddened by the dusty environment. They were subsequently employed for differential photometry.

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