TeV gamma - rays from photo - disintegration / de - excitation of nuclei in Westerlund 2

TeV gamma-rays can result from the photo-de-excitation of PeV cosmic ray nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HESS source at the edge of Westerlund 2. The UV background is provided by Lyman-alpha emission within the rich O and B stellar environment. The HESS flux results if there is efficient acceleration at the source of lower energy nuclei. The requirement that the Lorentz-boosted ultraviolet photons reach the Giant Dipole resonant energy (∼ 20 MeV) implies a strong suppression of the gamma-ray spectrum compared to an E γ behavior at energies below about 1 TeV. This suppression is not apparent in the lowest-energy Westerlund 2 datum, but will be probed by the upcoming GLAST mission. Two well-known mechanisms for generating TeV γ-rays in astrophysical sources are the purely electromagnetic (EM) synchrotron emission and inverse Compton scattering, and the hadronic (PION) one in which γ-rays originate from π production and decay. Very recently, we highlighted a third dynamic which leads to TeV γ-rays: photodisintegration of high-energy nuclei, followed by immediate photo-emission from the excited daughter nuclei [1]. For brevity, we label the photonuclear process A + γ → A + X , followed by A −→ A + γ-ray as “A”. Such a process may be operative in massive star formation regions with hot starlight. In this work we examine whether the A-process could be the origin of the very energetic γ-rays recently observed, with the High Energy Stereoscopic System (HESS) of Atmospheric Cherenkov Telescopes, from the young stellar cluster Westerlund 2 [2]. RCW 49 is a luminous cloud of ionized hydrogen located towards the outer edge of the Carina arm, at a distance d ≈ 8 kpc [3]. Embedded in RCW 49 is the massive star formation region Westerlund 2, hosting an extraordinary ensemble of hot OB stars; presumably at least a dozen early-type O stars, 100 B stars, and the remarkable Wolf-Rayet binary WR 20a [4]. For such a distance, the cluster core ∼ 5 results in a physical extent R ∼ 6 pc. The total mass of the stars within this region is found to be ≈ 4500M⊙. The total wind luminosity of all these O type stars has been estimated as ∼ 5 × 10 erg s, and the stellar luminosity of known massive stars is L = 2.15 × 10 erg s [5]. However, radio emission from the prominent giant HII region RCW 49 requires a larger luminosity of ionizing UV photons [6]. Indeed observations using the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope indicate that the total number of young stellar objects in this region is about 7000 [7]. Therefore, the above estimate of the total stellar luminosity should be taken as a lower bound. In the vicinity of WR20a, a clear excess of very high energy γ-rays was recently reported by the TEV GAMMA-RAYS FROM PHOTO-DISINTEGRATION/DE-EXCITATION OF NUCLEI IN WESTERLUND 2 H.E.S.S. Collaboration [2]. The significance of the excess is about 9σ. Compared to the point spread function of the instrument, the source (termed HESS J1023–2013575) appears slightly extended, corresponding to an intrinsic size of the γ-ray source of about 0.2; its center is slightly shifted compared to WR 20a. As expected for an extended source, the γ-ray flux is steady over time. By repeating the discussion of Cygnus OB2 [8], in what follows we obtain the expected γ-ray production through the A-process in Westerlund 2. To compute the photo-disintegration rate of a highly relativistic nucleus (with energy E = AEN = γAmN , where γ is the Lorentz factor) on starlight per nucleon [9],