On the Correlation between Infrared and 6.7 GHz Methanol Maser Emission in G36.705+0.096

ATel #15302; Bringfried Stecklum (TLS Tautenburg), Hendrik Linz (MPIA Heidelberg)
on 29 Mar 2022; 12:11 UT
Credential Certification: Bringfried Stecklum (stecklum@tls-tautenburg.de)

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The radiative excitation of Class II methanol masers implies that changes in the IR pumping radiation imprint on the maser flux (e.g., Cragg et al. 2005). This has been confirmed by recent accretion bursts from high-mass YSOs that were accompanied by maser flares (e.g., Fujisawa et al. ATel #8286 2015). For periodic maser sources, synchronized maser and IR emission was first revealed for the medium-mass YSO G107.298+5.639 (Stecklum et al. 2018). This behavior points to accretion periodicity in a very young binary system (e.g., Artymowicz & Lubow 1996). Thus, concurrent IR and maser flares trace the binarity of intermediate- and high-mass YSOs. Unfortunately, most of the YSOs hosting periodic methanol masers are saturated in space-based IR imaging, e.g., with NEOWISE.

This is not the case for G36.705+0.096, also known as IRAS18554+0319. Spitzer IRAC images show that it is embedded in an infrared dark cloud (Ellsworth-Bowers et al. 2015), hence it is likely located at the near-kinematic distance (3.4 kpc, Reid et al. 2014). Moreover, it has an EGO-like appearance. Its spectral energy distribution retrieved from the VizieR Photometry viewer implies a luminosity of 1.1E3 L☉. The low maser flux (< 10 Jy) might be related to intermediate luminosity. Early results of maser monitoring showed a period of 53 days (Sugiyama et al. 2015). To study whether this period also appears in the IR, NEOWISE photometry at 3.4 (W1) and 4.6 (W2) µm was obtained from IRSA. In addition, line-integrated fluxes from the Ibaraki 6.7 GHz Class II methanol maser database were used, kindly provided by Dr. Y. Yonekura. Since the Sugiyama et al. period rests on nine cycles only, a refinement was made by maximizing the amplitude of the phased maser fluxes. It was done by changing the period from 52.45 to 53.45 days with steps of 0.05 d. This yielded a period of 52.9±0.2 days. Since the NEOWISE time sampling is not suitable for identifying periods in this range, it has been tested whether the maser period matches the NEOWISE variability. In fact, the time series of the mean W1 and W2 magnitudes could be fitted using a sine function of this period, supplemented by constant, linear, and quadratic terms to account for temporal baseline trends.

Eventually, the correlation between the maser and the NEOWISE magnitudes was analyzed. For this purpose, mean maser fluxes from a time range centered on the NEOWISE epochs were compared to the mean W1 and W2 magnitudes. The time range was varied to maximize the significance of the correlation coefficient R. Furthermore, the probability that the correlation would occur by chance for a given number of data points was estimated. An range of ±3 days produces the most significant value of R = -0.75.

The link below provides a figure that shows (from left to right): Phased maser fluxes for the period given above, the magnitude-maser flux relation (where the W1 scale has been matched to that of W2), and the significance plot for the correlation coefficient.

G36.705+0.096 is another example of a YSO with simultaneous IR and methanol maser flares, likely caused by binary accretion modulation. With the ongoing monitoring of the IR and maser emission, the number of these objects is increasing (e.g., Olech et al. 2022). This will help to advance our knowledge on the binarity of massive young stars.

This ATel summarizes our independent research on the maser-IR correlation in G36.705+0.096 which is also the topic of a forthcoming paper of Uchiyama, Yonekura & Tanabe (ApJ, submitted).

Artymowicz, P. & Lubow, S. H. 1996, ApJ, 467, L77
Cragg, D. M. et al. 2005, MNRAS, 360, 533
Ellsworth-Bowers, T. P. et al. 2015, ApJ, 799, 29
Fujisawa, K. et al. 2015, ATel #8286
Olech, M. et al. 2022, arXiv: 2203.10859
Reid, M. J. et al. 2014, ApJ, 783, 130
Stecklum, B. et al. 2018, IAUS, 336, 37
Sugiyama, K. et al. 2015, PKAS, 30, 129

Fig. 1