[Paper Review] Gamma-rays from massive protostars

Gustavo E. Romero, Anabella Araudo+2 more|ArXiv.org|Aug 6, 2009

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper proposes that massive protostars driving high-velocity bipolar outflows can produce detectable GeV–TeV gamma-ray emission via relativistic particle acceleration at terminal shocks and subsequent interactions in dense molecular clouds. Using refined shock acceleration and radiation transfer models, it predicts that Fermi and Cherenkov telescopes could detect such sources, especially through diffuse emission from cosmic-ray diffusion, with luminosities up to ~10³² erg s⁻¹.

ABSTRACT

Massive protostars have associated bipolar outflows with velocities of hundreds of km/s. Such outflows produce strong shocks when interact with the ambient medium leading to regions of non-thermal radio emission. Under certain conditions, the population of relativistic particles accelerated at the terminal shocks of the protostellar jets can produce significant gamma-ray emission. We estimate the conditions necessary for high-energy emission in the non-thermal hot spots of jets associated with massive protostars embedded in dense molecular clouds. Our results show that particle-matter interactions can lead to the detection of molecular clouds hosting massive young stellar objects by the Fermi satellite at MeV-GeV energies and even by Cherenkov telescope arrays in the GeV-TeV range. Astronomy at gamma-rays can be used to probe the physical conditions in star forming regions and particle acceleration processes in the complex environment of massive molecular clouds.

Motivation & Objective

• To investigate the conditions under which massive protostars can produce high-energy gamma-ray emission detectable by Fermi and Cherenkov telescopes.
• To model particle acceleration and radiation processes in the terminal shocks of protostellar jets within dense molecular clouds.
• To assess the contribution of proton-proton collisions and inverse Compton scattering to the overall gamma-ray luminosity.
• To evaluate the detectability of such sources by current and future gamma-ray observatories, including sensitivity to diffuse emission from cosmic-ray diffusion.

Proposed method

• Modeling diffusive shock acceleration (DSA) at forward and reverse shocks in protostellar jet outflows using relativistic particle injection and magnetic field strength estimates.
• Calculating gamma-ray emission from inelastic proton-proton collisions and inverse Compton scattering of infrared photons by relativistic electrons.
• Including particle escape timescales and diffusion of relativistic protons into the surrounding molecular cloud to produce extended, diffuse gamma-ray emission.
• Using spectral energy distribution (SED) modeling to compare predicted emission with Fermi and Cherenkov telescope sensitivities.
• Applying physical parameters from observed sources (IRAS 16547-4247 and HH 80-81) such as jet luminosity, cloud density, magnetic field, and shock velocity.
• Estimating total gamma-ray luminosities for different particle spectral indices (a = 0 for leptonic, a = 100 for hadronic) and comparing with observational thresholds.

Experimental results

Research questions

• RQ1 Under what conditions can relativistic particles accelerated in protostellar jet shocks produce gamma-ray emission detectable by Fermi?
• RQ2 How does cosmic-ray diffusion from radio lobes into the surrounding molecular cloud contribute to extended gamma-ray emission?
• RQ3 What is the expected gamma-ray luminosity from massive protostars, and can it reach detectable levels for current and future Cherenkov telescopes?
• RQ4 How do the relative contributions of leptonic (IC, synchrotron) and hadronic (pp collisions) processes shape the observed gamma-ray spectrum?
• RQ5 Can the detection of gamma-ray cutoffs in the SED constrain shock velocity and particle acceleration efficiency?

Key findings

• The model predicts gamma-ray luminosities up to ~10³² erg s⁻¹ for massive protostars, primarily from hadronic processes in dense molecular clouds.
• Diffuse gamma-ray emission from cosmic-ray diffusion into the cloud can increase the integrated luminosity to ~5×10³² erg s⁻¹ in the 0.1–100 GeV range and ~6×10³² erg s⁻¹ above 100 GeV.
• For sources like IRAS 16547-4247 and HH 80-81, the predicted gamma-ray flux at >100 GeV reaches ~0.01 Crab, detectable by Cherenkov telescopes with ~50 hours of observation.
• The leptonic case (a = 0) yields luminosities below 10³⁰ erg s⁻¹, making it undetectable with current instruments, while the hadronic case (a = 100) produces strong, potentially detectable emission.
• The detection of spectral cutoffs in gamma-ray SEDs could constrain shock velocity and diffusion coefficients, providing insights into particle acceleration mechanisms.
• Cosmic-ray re-acceleration via magnetic turbulence may further enhance source luminosity, increasing detectability in future surveys.