New Methanol Maser Transitions and Maser Variability Identified from an Accretion Burst Source G358.93-0.03

Abstract

The high-mass young stellar object G358.93-0.03 underwent an accretion burst during the period from 2019 January to June. Given its extraordinary conditions, a number of new maser transitions may have been naturally excited during the burst stage. Searching for new maser lines and monitoring maser variability associated with the accretion burst event are important for understanding the complex conditions of the massive star formation toward G358.93-0.03. In this work, using the Shanghai 65 m Tianma Radio Telescope, we continuously monitored the multiple maser (including methanol and water) transitions toward G358.93-0.03 during the burst in the period from 2019 March 14 to May 20. There were 23 CH3OH maser transitions and one H2O maser transition detected from the monitoring. Nearly all the detected maser transitions toward this source have dramatic variations in their intensities within a short period of ∼2 months. Eight new methanol transitions from G358.93-0.03 were identified to be masering in our observations based on their spectral profile, line width, intensity, and the rotation diagram. During the monitoring, the gas temperature of the clouds in the case of saturated masers can show a significant decline, indicating that the maser clouds were going through a cooling process, possibly associated with the propagation of a heat wave induced by the accretion burst. Some of the maser transitions were even detected with the second flares in 2019 April, which may be associated with the process of the heat-wave propagation induced by the same accretion burst acting on different maser positions.

Physical Environments of the Luminosity Outburst Source NGC 6334I Traced by Thermal and Maser Lines of Multiple Molecules

“…The maser decay trend suggests that the outburst may not have the expected long duration (more than 40 yr) from Hunter et al (2021). By contrast, the 6.7 GHz maser emission increased by 2 orders of magnitude from <10 Jy in early observations to ∼1000 Jy in 2019 mid-March (see Chen et al 2020b and references therein), and started a rapid decay since 2019 April; thus, it has a short flare duration of half a year (e.g., Miao et al 2022). So the 6.7 maser flare is more active in NGC 6334I MM1 than in G358.93-0.03, which is also consistent with a stronger increase in both the luminosity and accretion energy in NGC 6334I MM1 (see Hunter et al 2021).…”

“…For example, as summarized in Stecklum et al (2021), the total accretion energy (E acc ) of NGC 6334I MM1 is 3.2 × 10 46 erg, which is 3 times higher than that of S255IR- NIRS3 (1.2 × 10 46 erg), and 1 order of magnitude higher than that of G358.93-0.03 (2.9 × 10 45 erg). Moreover, the flare event in the NGC 6334I MM1 prostar has a long duration of 40-130 yr (Hunter et al 2021), while that in S255IR-NIR3 has a duration of a few years (Carattio Garatti et al 2017), and G358.93-0.03 has only about half a year traced by monitoring observations of multiple methanol maser transitions (e.g., Miao et al 2022). These distinctions imply that the magnitudes and durations of outburst events differ between the three luminosity outburst sources.…”

“…Conversely, abundant class I maser (including 11 lines) and quasi-thermal transitions (including 16 lines) were detected toward NGC 6334I from the TMRT survey and were likely associated with the MM1 outburst. Only two well-known class I transitions at 36.169 and 44.069 GHz were detectable in G358.93-0.03 (see Miao et al 2022).…”

“…It is worth mentioning that a direct link between the 6.7 GHz class II CH 3 OH maser flaring and accretion burst has recently been established only in a small number of HMYSOs, e.g., G358.93-0.03-MM1 (Breen et al 2019;MacLeod et al 2019;Sugiyama et al 2019;Burns et al 2020), S255IR-NIRS3 (Fujisawa et al 2015;Carattio Garatti et al 2017;Moscadelli et al 2017), and NGC 6334I-MM1 (Hunter et al 2017(Hunter et al , 2018MacLeod et al 2018). Besides, 22 GHz H 2 O (water) maser emission from protostars has exhibited flare phenomena toward Orion KL (Abraham et al 1981;Omodaka et al 1999;Hirota et al 2014), W49N (Liljeström & Gwinn 2000;Honma et al 2004), G25.65 + 1.05 (Volvach et al 2017a(Volvach et al , 2017bLekht et al 2018), G358.93-0.03 (Bayandina et al 2022;Miao et al 2022), and W51D (Zhang et al 2022).…”

“…During the accretion burst stage, new maser species may be energized in exotic or rare physical conditions induced by accretion bursts, corresponding to the best epoch to search for and identify new maser species. More than 30 new methanol maser transitions have been detected toward a short-lived (within a few months) accretion burst source G358.93-0.03 (Breen et al 2019;Brogan et al 2019;MacLeod et al 2019;Miao et al 2022), and some masers from new species (such as CH 3 OH, HDO, and HNCO) have also been detected toward this source (Chen et al 2020a(Chen et al , 2020b. The latter three new species of masers provide solid contributions to the episodic accretion scheme because they accurately trace spiral-arm accretion flow structures caused by the gravitational instability of large-mass disks (Chen et al 2020b).…”