The Unexpected Findings of an Emission Shell

Title: Discovery of a [O III] Emission Shell Around the X-ray Binary CI Cam

Authors: Robert A. Fesen, Marcel Dreschsler, Nicolas Martino, and Yann Sainty

First Author’s Institution: Department of Physics and Astronomy, Dartmouth College, NH, U.S.A

Status: To be submitted AAS Journals

From Be Star to X-ray Binary

Among the several spectral classes of normal stars that we know and love, there is one particularly eccentric type called the Be stars. Unfortunately, no, the ‘e’ there does not stand for eccentric. Much like the B-type stars, Be stars are incredibly hot and massive objects. However, what sets them apart is the notable presence of excess emission features in their spectral profiles, in contrast with the absorption lines typically seen in the spectra of regular B stars. Thus, the ‘e’ stands for emission.

Figure 1: Artist’s illustration of an X-ray binary system that consists of a normal star and a compact object. The gravity of the compact object pulls matter off the normal star creating an accretion disk, shown in blue. Credit: ESA, NASA, and Felix Mirabel (French Atomic Energy Commission and Institute for Astronomy and Space Physics/Conicet of Argentina)

Our star today, CI Camelopardalis (CI Cam), was initially cataloged as a Be star, but when it was later found that the spectrum was showing particularly the forbidden emission lines, it was reclassified as a B[e] star. The brackets designate the forbidden lines that come from an improbable transition between metastable energy levels in atoms or ions, which can only be observed in low-density astrophysical environments a (for example, the doubly ionized oxygen forbidden line is written as [O III]).

CI Cam underwent a dramatic X-ray outburst in 1998, which not only revealed the presence of an unseen compact companion, but also firmly established it as an X-ray binary system. An X-ray binary is a fascinating duo of a normal star and a compact object, which could be a white dwarf, a neutron star, or a black hole (see Figure 1). The two orbit closely around a common center of mass, emitting intense X-ray radiation. This high-energy emission is produced by the accretion process where material from the star gets pulled onto the compact object by its immense gravitational field. As this material spirals inward, it heats up to temperatures in the millions of degrees, causing it to radiate very brightly in the X-ray part of the electromagnetic spectrum. Interestingly, the exact nature of CI Cam’s compact companion remains unclear to this day.

The Discovery

In a serendipitous finding, today’s authors uncovered a previously unknown shell of emitting gas surrounding the CI Cam system. This shell, spanning about 8 arcminutes by 12 arcminutes on the sky, is brightest in the forbidden [O III] emission line at 5007 Angstrom (within the optical or visible light regime). The discovery unfolded from the optical data of the large supernova remnant G150.3+4.5 (G150) that was observed as part of a limited wide-field imaging survey led by Robert Fesen from Dartmouth College (the first author of today’s paper) of optical emission associated with Galactic remnants. The emission shell was spotted adjacent to the remnant’s filaments along its northwestern edge (see Figure 2).

Figure 2: Composite image showing the emission shell around CI Cam and the western portion of the Galactic supernova remnant G150.3+4.5. Blue represents the [O III] emission and red represents the Hα emission. Figure 1 (modified from top left and bottom left panels) in the paper.

The presence of [O III] emission indicates the existence of extremely hot ionizing sources that can provide the high-energy photons needed to doubly ionize oxygen atoms. Typically, strong [O III] emissions are seen to have filamentary or arc-like structures tracing the interfaces between ionized and neutral gas regions. These emission can also be enhanced by the presence of shocks, in addition to the photoionization by hot sources. An emission shell featuring strong [O III] emission around CI Cam with faint emissions (see Figure 3), implies the presence of high-velocity shocks of about 90 to 150 km/s, potentially driven by CI Cam’s outflows into the surrounding interstellar medium.

Figure 3: Monochromatic images of the emission shell around CI Cam as seen in [O III] (top) and in Hα (bottom). Figure 1 (top right panels) in the paper.

By adopting CI Cam’s newly refined Gaia distance of 4.1 kiloparsecs, the physical size of the shell, assuming it is indeed the result of CI Cam’s outflows, spans approximately 9.5 x 14.3 parsecs. To put this into perspective, the longer side of the nebula is roughly equivalent to a hundred billion times the distance between New York City and Los Angeles via the I-40. This dimension is comparable to the dimensions of nebulae observed around Wolf-Rayet stars, which are known to generate such large structures through their powerful stellar winds and outflows.

Due to the similarity in [O III] brightness between the shell and the G150’s filaments, their close proximity raised a question of a potential physical association between the two objects. However, the authors found no direct evidence in the morphology linking the shell to the remnant on the imaging data. They also took into account the available distance estimates of G150 derived from radio and γ-ray observations, which suggested a much closer distance compared to CI Cam’s. The authors argued that if G150 were actually at CI Cam’s distance, it would have to be much larger and ancient supernova remnant, which seems highly unlikely given its bright optical filaments that are suggestive of a relatively young age. The authors then concluded that the apparent proximity of the shell to G150 is likely just a chance alignment to Cl Cam.

While X-ray binaries are primarily studied in the X-ray, optical observations offer a complementary perspective, helping us to understand the details of the binary’s interactions with its surrounding medium. The size of the emission shell around CI Cam that is comparable to those created by powerful Wolf-Rayet stars provides evidence for the strong stellar winds and the transient outburst that CI Cam has unleashed over an extended period. This, in turn, promises to shed light on the mass loss mechanisms and accretion processes in this class of objects. This discovery not only highlights the significance of wide-field imaging surveys in unveiling previously unknown structures around peculiar objects, but also opens up a new observational window for advancing our understanding of the impact that such systems have on enriching and shaping their cosmic neighborhoods.

Astrobite edited by Ivey Davis

Featured image credit: Figure 1 bottom left of the paper. 

About Janette Suherli

Janette is a PhD student at University of Manitoba in Winnipeg, Canada. Her research focuses on the utilization of integral field spectroscopy for the studies of supernova remnants and their compact objects in the optical. She is also the current chair of Graduate Student Committee for the Canadian Astronomical Society (CASCA). She grew up in Indonesia where it is summer all year round! Before pursuing her PhD in astrophysics, Janette worked as a data analyst for a big Indonesian tech company, combating credit card fraud.

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