Discovery of ammonia (9,6) masers in two high-mass star-forming regions

Abstract

Context. Molecular maser lines are signposts of high-mass star formation, probing the excitation and kinematics of very compact regions in the close environment of young stellar objects and providing useful targets for trigonometric parallax measurements. Aims. Only a few NH3 (9,6) masers are known so far, and their origin is still poorly understood. Here we aim to find new NH3 (9,6) masers to provide a better observational basis for studying their role in high-mass star-forming regions. Methods. We carried out NH3 (9,6) observations toward Cepheus A and G34.26+0.15 with the Effelsberg 100-meter telescope (beam size 49′′) and the Karl G. Jansky Very Large Array (JVLA; beam size about 1′′.2). Results. We discovered new NH3 (9,6) masers in Cep A and G34.26+0.25, which increases the number of known high-mass star-forming regions hosting NH3 (9,6) masers from five to seven. Long-term monitoring (20 months) at Effelsberg shows that the intensity of the (9,6) maser in G34.26+0.25 is decreasing, while the Cep A maser remains stable. Compared to the Effelsberg data and assuming linear variations between the epochs of observation, the JVLA data indicate no missing flux. This suggests that the NH3 (9,6) emission arises from single compact emission regions that are not resolved by the interferometric measurements. As JVLA imaging shows, the NH3 (9,6) emission in Cep A originates from a sub-arcsecond-sized region, slightly to the west (0′′.28 ± 0′′.10) of the peak position of the 1.36 cm continuum object, HW2. In G34.26+0.25, three NH3 (9,6) maser spots are observed: one is close to the head of the cometary ultracompact H II region C, and the other two are emitted from a compact region to the west of the hypercompact H II region A. Conclusions. The newly found (9,6) masers appear to be related to outflows. The higher angular resolution of JVLA and very long baseline interferometry observations are needed to provide more accurate positions and constraints for pumping scenarios.

Discovery of non-metastable ammonia masers in Sagittarius B2

“…Overall, the detected non-metastable ammonia masers in Sgr B2 are consistent with the discussion on pumping scenarios in Yan et al (2022). Therefore, we speculate that the detected NH 3 masers in the non-metastable (6,3), (7,4), (8,5), (9,6), and (10,7) transitions in Sgr B2(N), Sgr B2(NS), Sgr B2(M), and Sgr B2(S) appear to be associated with shocks caused either by outflows or UCHII expansion.…”

“…This indicates that the presence of hot, dense gas alone is not sufficient to excite these masers. The ammonia masers detected in Sgr B2(S) are close to the head of the cometary UCH ii region, similar to the NH 3 (9,6) maser M1 in G34.26+0.15 (Yan et al 2022). The maser spots in Sgr B2(S) show almost twice the angular distance compared to M1, with an offset of (+0 .…”

“…11) in G34.26+0.15. In view of the different distances of G34.26+0.15 (D ∼ 3.3 kpc Yan et al 2022) and Sgr B2 (8.2 kpc see Sect.1), the linear distance is even five times larger in Sgr B2(S), that is 0.03 pc. The velocity difference between the masers and the cometary UCH ii regions is ten times higher in Sgr B2(S) than in G34.26+0.15 (∆V SgrB2(S) ≥17.4 km s −1 and ∆V G34.26 ∼-1.3 km s −1 ).…”

“…Since their discovery in the (J, K) = (3,3) metastable (J = K) line toward the high-mass star-forming region (HMSFR) W33 (Wilson et al 1982), sources emitting maser emission in the ammomia molecule (NH 3 ) have attracted much attention. To date, metastable ammonia maser lines have been detected in 22 HMS-FRs (see Yan et al 2022, and references therein), while nonmetastable (J > K) ammonia masers have only been found in ten sources. The NH 3 (6,3), (7,4), (8,5), and (9,6) maser lines, which will be discussed below, arise from energy levels of 551 K, 713 K, 892 K, and 1089 K above the ground state, respectively.These four maser transitions have only been detected, respectively, in three (NGC7538, W51, and Sgr B2(N)), in two (W51 and Sgr B2(N)), in two (W51 and Sgr B2(N)), and in seven (W51, NGC7538, W49, DR21 (OH), Sgr B2(N), Cep A and G34.26+0.15) HMSFRs (Madden et al 1986;Henkel et al 2013;Mei et al 2020;Yan et al 2022).…”

“…To date, metastable ammonia maser lines have been detected in 22 HMS-FRs (see Yan et al 2022, and references therein), while nonmetastable (J > K) ammonia masers have only been found in ten sources. The NH 3 (6,3), (7,4), (8,5), and (9,6) maser lines, which will be discussed below, arise from energy levels of 551 K, 713 K, 892 K, and 1089 K above the ground state, respectively.These four maser transitions have only been detected, respectively, in three (NGC7538, W51, and Sgr B2(N)), in two (W51 and Sgr B2(N)), in two (W51 and Sgr B2(N)), and in seven (W51, NGC7538, W49, DR21 (OH), Sgr B2(N), Cep A and G34.26+0.15) HMSFRs (Madden et al 1986;Henkel et al 2013;Mei et al 2020;Yan et al 2022). The NH 3 (10,7) line, also observed by us, connects states as high as 1303 K above the ground state and was so far only classified as a maser in W51 (Henkel et al 2013).…”