Thermal emission from bow shocks I: 2D Hydrodynamic Models of the Bubble Nebula

Abstract

The Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical wind-blown bubble around the relatively unevolved O star BD+60 • 2522. The small dynamical age of the nebula and significant space velocity of the star suggest that the Bubble Nebula might be a bow shock. We have run 2D hydrodynamic simulations to model the interaction of the central star's wind with the interstellar medium (ISM). The models cover a range of possible ISM number densities of n = 50 − 200 cm −3 and stellar velocities of v * = 20 − 40 km s −1 . Synthetic Hα and 24 µm emission maps predict the same apparent spherical bubble shape with quantitative properties similar to observations. The synthetic maps also predict a maximum brightness similar to that from the observations and agree that the maximum brightness is at the apex of the bow shock. The best-matching simulation had v * ≈ 20 km s −1 into an ISM with n ∼ 100 cm −3 , at an angle of 60 • with respect to the line of sight. Synthetic maps of soft (0.3 − 2 keV) and hard (2 − 10 keV) X-ray emission show that the brightest region is in the wake behind the star and not at the bow shock itself. The unabsorbed soft X-rays have luminosity ∼ 10 32 − 10 33 erg s −1 . The hard X-rays are fainter, luminosity ∼ 10 30 − 10 31 erg s −1 , and may be too faint for current X-ray instruments to successfully observe. Our results imply that the O star creates a bow shock as it moves through the ISM and in turn creates an asymmetric bubble visible at optical and infrared wavelengths, and predicted to be visible in X-rays. The Bubble Nebula does not appear to be unique, it could be just a favourably oriented very dense bow shock. The dense ISM surrounding BD+60 • 2522 and its strong wind suggest that it could be a good candidate for detecting non-thermal emission.

The Bubble Nebula NGC 7635 – testing the wind-blown bubble theory

“…Accordingly, BD+60 • 2522 is found to move along an angle of 46 • with respect to the plane of the sky with a space velocity of 36 km s −1 . The angle is close to that of 56 • estimated by Green et al (2019), but the space velocity is notably larger than their space velocity of 20 km s −1 .…”

“…We can estimate upper limits to the electron densities of the hot gas in the Bubble Nebula by using the normalisation parameters obtained with PIMMS (see Section 4.1). Adopting a plasma temperature of 0.22 keV and 2.2 keV (as those predicted by Green et al 2019), we estimate upper limits of n e <0.07 cm −3 and n e <0.04 cm −3 , which are very low com-pared to the detections of soft X-ray emission from other WBBs. Green et al (2019) argue that some structures around runaway stars, in particular hydrodynamical instabilities, might be suppressed by magnetic fields.…”

“…Adopting a plasma temperature of 0.22 keV and 2.2 keV (as those predicted by Green et al 2019), we estimate upper limits of n e <0.07 cm −3 and n e <0.04 cm −3 , which are very low com-pared to the detections of soft X-ray emission from other WBBs. Green et al (2019) argue that some structures around runaway stars, in particular hydrodynamical instabilities, might be suppressed by magnetic fields. This would certainly reduce the efficiency of mixing into the hot bubble causing a low X-ray flux, but we note that magnetic fields have not been reported to be present in the Bubble Nebula so far.…”

“…Alternatively, Green et al (2019) proposed that the Bubble Nebula is a bow shock around a runaway star, following the reported high proper motions of BD+60 • 2522 (28±3 km s −1 ) from the Gaia data release. As these authors propose, the Bubble Nebula would not be a simple WBB, but rather "a favourably oriented dense bow shock" located at the north of the star.…”

“…In Green et al (2019)'s models, the hot bubble becomes unstable at the wake of the bow shock producing hydrodynamical instabilities that are then a source of mass, mixing material into the hot bubble to produce optimal conditions for soft X-ray emission. Green et al (2019) estimated that such process would produce an unabsorbed X-ray luminosity for the soft X-ray XMM-Newton band (0.3-2.0 keV) of L X =10 32 -10 33 erg s −1 and a corresponding estimate for the hard X-rays of 10 30 -10 31 erg s −1 . Their estimated mean temperature of the soft and hard X-ray emission are 0.22 keV and 2.2 keV, respectively.…”