tokens: tips of the H 2 bow shocks, where the shock velocities are high and the gas will be dissociated, rather than behind the H 2 bow shocks (e.g., Davis et al. 1999 , 2000 ; López et al. 2010 ; Lee et al. 2014 ; Bally et al. 2015 ). Detection of further [Fe ii ] emission in G53.11_MM1 with high signal-to-noise ratio requires deeper imaging observations, although the marginal detection of [Fe ii ] emission can be interpreted as intrinsically fainter or absent [Fe ii ] emission compared to H 2 emission. In the outflows from high-mass YSOs, the [Fe ii ] detection rate with respect to the H 2 detection rate tends to be low, ≲50% or much less (Cooper et al. 2013 ; Wolf-Chase et al. 2013 ; Caratti o Garatti et al. 2015 ). The brightness of the [Fe ii ] lines also tend to be weaker than the brightness of the H 2 lines (Caratti o Garatti et al. 2015 ). This can be attributed to the different extinction effect between the two lines but it is more likely to happen because H 2 and [Fe ii ] emission arise from different physical conditions, such as gas density, temperature, or shock velocity. In the G53.11_MM1 outflow, the strong, extended H 2 emission with very weak or negligible [Fe ii ] emission may imply that slow, C-type shocks are dominant in G53.11_MM1. Further spectroscopic observations will be necessary to derive the physical conditions of the environment and confirm the shock’s properties. 5. Central YSOs The core G53.11_MM1 is bright from IR to millimeter, with a large IR-excess emission. The central star (YSO1) was previously identified in the Red MSX Source survey with a bolometric luminosity of (3–4) × 10 3 L ⊙ (G053.1417+00.0705; Mottram et al. 2011 ; Lumsden et al. 2013 ); however, it is in fact composed of two sources, YSO1 and YSO2, which are separated by ∼8′′ in the Spitzer mid-IR images with higher spatial resolution. Both YSOs are saturated in the MIPS 24 μ m image but their SEDs with strong excess in mid-IR and spectral indices derived by using the available photometry from the GLIMPSE and MSX catalogs classify them as Class I YSOs that have a dusty envelope infalling onto a central protostar (Kim et al. 2015 ). The two YSOs are also observed in near-IR wavebands. Both are fairly bright in the K -band, marginally detected in the H -band and are not observed in the J -band, which indicates that they are deeply embedded. The evolutionary stages of the YSOs, with the proximity to the center of the outflow (see Figure 2 ), suggest that one or both can be the driving source of the G53.11_MM1 outflow. The coordinates of YSO1 and YSO2 are ( α 2000 , δ 2000 ) = (19 h 29 m 1760, +17°56′233) and ( α 2000 , δ 2000 ) = (19 h 29 m 1726, +17°56′173), respectively. In the following, we will discuss their photometric variability in near-IR and physical parameters constrained from SED analysis. 5.1. Near-IR Photometric Variability YSOs are known to commonly show variability (e.g., Carpenter et al. 2001 , 2002 ; Morales-Calderón et al. 2011 ; Johnstone et
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