Using Spectroscopy to Study B-Type Stars in Open Cluster NGC 6834

A recent astronomical study saw a research team from Argentina study an open star cluster using spectroscopy instrumentation. This study, which was published in Monthly Notices of the Royal Astronomical Society, was led by M. A. Ruiz Diaz from the Universidad Nacional de La Plata (UNLP) and Instituto de Astrofísica La Plata. In the study, Ruiz Diaz and the team investigated the open star cluster NGC 6834 to learn more about its fundamental properties. This study offers another step into using spectroscopy in space exploration applications to better understand the mysteries of the galaxy.

The Pleiades deep space photography a majestic star cluster. Generated by AI. | Image Credit: © Terry - stock.adobe.com

What is NGC 6834 and why is it important?

NGC 6834 is an intermediate-age open cluster located in the constellation Cygnus (1,2). Cygnus is a constellation in the northern sky, and it is known as the Swan constellation (3). It has 10 stars with known planets, and it is the 16^th-largest constellation in the night sky (3). Situated north of the celestial equator, the NGC 6834-star cluster is estimated to be between 40 and 65 million years old (2). This age range makes it particularly interesting for astrophysical studies because it is expected to host a robust population of B-type stars, including Be stars, which are fast-rotating B stars with emission lines caused by circumstellar gas disks (1). Previous observations had identified 15 B-type variable stars in the region, with four confirmed as Be stars through spectroscopy (1).

How did the Research Team Study the NGC 6834 Open Cluster in their Study?

In their study, Ruiz Diaz and colleagues undertook a comprehensive spectrophotometric survey using data from the Gemini North telescope in Hawaiʻi. The study combined photometric imaging using gri filters and a large spectroscopic data set acquired through the Gemini Multi-Object Spectrograph (GMOS) in both direct and multi-object modes (1).

What did their analysis entail?

The analysis conducted by the research team included 109 optical spectra of 89 stars in and around the cluster. Many of these stars had never been observed spectroscopically (1). Using these data, the researchers derived key stellar parameters for the first time, including effective temperature, surface gravity, absolute visual magnitude, bolometric correction, and projected rotational velocity for 64 B-type stars (1).

What are the key takeaways from this study?

There are several key takeaways that spectroscopists and astronomers should take from this study. The first takeaway is that the study determined the cluster’s average foreground color excess, E(B-V) = 0.62, which is a value corroborated by both photometric and spectroscopic methods (1). This parameter is essential for correcting interstellar reddening and thus for accurately placing stars in the Hertzsprung-Russell diagram, which helps determine stellar ages and evolutionary stages (1).

The second key takeaway is that through analyzing the spectra, the researchers were able to uncover five unreported Be stars. Among these five stars, NGC 6834 3 and NGC 6834 9 were found to exhibit strong Hα emission lines, identifying them as new Be stars of spectral types B3V and B5V, respectively (1). These findings are particularly significant because they expand the known population of Be stars within the cluster and provide new insights into their transient nature (1).

For the other three stars, two of them, NGC 6834 23 and NGC 6834 121, were identified as non-members of the cluster, while the other one, NGC 6834 118, was confirmed as a cluster member.

What is the significance of this study?

This study is significant because it provides more spectroscopic data for this star cluster. Despite the broad reach of major sky surveys such as LAMOST and SDSS, fewer than ten low-mass stars in this region had been observed spectroscopically (1). The present study therefore fills a significant data gap, particularly for B-type stars.

The research team also provided new information about the rotational dynamics of B-type stars. The determined projected rotational velocities for 64 B stars are essential for probing the angular momentum distribution and formation history of stars within the NGC 6834 open cluster, which provided an expanded census of B and Be stars and the precise characterization of their physical properties (1).

References

1. Ruiz Diaz, M. A.; Aidelman, Y. J.; Baume, G.; Granada, A. A comprehensive photometric and spectroscopic study of B-type stars in the open cluster NGC 6834. Mon. Not. R. Astron. Soc. 2025, 537 (2), 1763–1778. DOI: 10.1093/mnras/staf114
2. The Sky Live, NGC 6834 – Open Cluster in Cygnus. The Sky Live. Available at: https://theskylive.com/sky/deepsky/ngc6834-object (accessed 2025-05-12).
3. Constellation Guide, Cygnus Constellation. Constellation Guide. Available at: https://www.constellation-guide.com/constellation-list/cygnus-constellation/#:~:text=Its%20name%20means%20%E2%80%9Cthe%20swan,known%20as%20the%20Northern%20Cross. (accessed 2025-05-12).