[Paper Review] Infrared observations of the flaring maser source G358.93-0.03 -- SOFIA confirms an accretion burst from a massive young stellar object

B. Stecklum, V. Wolf+25 more|arXiv (Cornell University)|Jan 5, 2021

Astrophysics and Star Formation Studies102 references61 citations

TL;DR

This study confirms an accretion burst in the massive young stellar object (MYSO) G358.93-0.03 using SOFIA/FIFI-LS far-infrared observations, which detected a significant luminosity increase in the dust continuum emission. The burst, the shortest and least luminous known to date, triggered a flaring Class II methanol maser, validating maser flares as reliable indicators of episodic accretion in high-mass star formation.

ABSTRACT

Class II methanol masers are signs of massive young stellar objects (MYSOs). Recent findings show that MYSO accretion bursts cause flares of these masers. Thus, maser monitoring can be used to identify such bursts. Burst-induced SED changes provide valuable information on a very intense phase of high-mass star formation. In mid-January 2019, a maser flare of the MYSO G358.93-0.03 was reported. ALMA and SMA imaging resolved the core of the star forming region and proved the association of the masers with the brightest continuum source MM1. However, no significant flux rise of the (sub)mm dust continuum was found. Thus, we performed NIR imaging with GROND and IFU spectroscopy with FIFI-LS aboard SOFIA to detect possible counterparts to the (sub)mm sources, and compare their photometry to archival measurements. The comparison of pre-burst and burst SEDs is of crucial importance to judge whether a luminosity increase due to the burst is present and if it triggered the maser flare. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our RT modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the multi-epoch SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous MYSO accretion burst so far.

Motivation & Objective

• To verify the presence of an accretion burst in the massive young stellar object (MYSO) G358.93-0.03, which was inferred from flaring Class II methanol masers.
• To determine whether the maser flare was driven by a luminosity increase in the central protostar, using near- and far-infrared photometry.
• To constrain the burst parameters—such as peak luminosity, duration, and energy—through radiative transfer modeling of multi-epoch spectral energy distributions (SEDs).
• To investigate the evolutionary timescales of heating and cooling in the circumstellar environment following the burst.
• To explore the connection between burst-induced changes in the protostellar envelope and the observed maser relocation and flaring.

Proposed method

• Acquired near-infrared (NIR) imaging with the GROND instrument on the 2.2-m MPG/ESO telescope to measure pre-burst and burst fluxes at short wavelengths.
• Conducted far-infrared (FIR) integral-field spectroscopy with SOFIA’s FIFI-LS instrument at two epochs (early burst and ~16 months post-burst) to measure fluxes from the MM1 source at 889–1532 µm.
• Compared the observed FIR fluxes with archival data from Herschel and ALMA/SMA to detect significant flux increases indicative of a luminosity rise.
• Constructed pre-burst, burst, and post-burst SEDs using multi-wavelength photometry, excluding contributions from other sources via deconvolution techniques.
• Perfomed radiative transfer modeling of the dust continuum SEDs to derive burst parameters such as luminosity, duration, and mass accretion rate.
• Used interferometric (ALMA/SMA) data to isolate the MM1 source and confirm its association with the maser flaring, enabling accurate flux deblending.

Experimental results

Research questions

• RQ1 Did the flaring of the 6.7 GHz methanol maser in G358.93-0.03 correspond to a detectable increase in the infrared luminosity of the central MYSO?
• RQ2 What are the key parameters (luminosity, duration, energy) of the accretion burst, as derived from multi-epoch SED modeling?
• RQ3 How do the heating and cooling timescales of the circumstellar envelope compare with theoretical expectations following a burst?
• RQ4 What is the role of a transient circumstellar disk in the burst dynamics and maser reactivation?
• RQ5 To what extent do maser flares serve as reliable tracers of episodic accretion in massive star formation?

Key findings

• SOFIA/FIFI-LS detected a significant increase in FIR flux from the MM1 source, with fluxes exceeding Herschel measurements by a factor of ~2.5 at 889 µm, confirming a luminosity rise.
• The burst luminosity reached ~1.5 × 10⁴ L⊙ at peak, with a duration of ~1.5 years, making it the shortest and least luminous accretion burst known to date.
• Radiative transfer modeling of the SEDs revealed a transient circumstellar disk around the MYSO, which may have played a key role in modulating the burst.
• The heating timescale of the envelope was estimated to be ~0.5 years, while the cooling timescale was ~1.5 years, consistent with models of burst-driven heating in massive protostars.
• The maser flaring and outward propagation of maser spots were directly linked to the burst’s thermal radiation front, confirming the radiative pumping mechanism.
• The post-burst SED still showed elevated fluxes 16 months after the peak, indicating a prolonged thermal relaxation phase in the circumstellar environment.