An Initial Overview of the Extent and Structure of Recent Star Formation within the Serpens Molecular Cloud Using Gaia Data Release 2

Abstract

The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure ∼ 1167 kinematic members of Serpens, few of which were previously identified, to evaluate the star formation history of the complex. The optical members of Serpens are concentrated in three distinct groups located at 380-480 pc; the densest clusters are still highly obscured by optically-thick dust and have few optical members. The total population of young stars and protostars in Serpens is at least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may be far higher. Distances to dark clouds measured from deficits in star counts are consistent with the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila Rift, dark clouds located at 600-700 pc, and behind patchy extinction, here called the Serpens Cirrus, located at ∼ 250 pc. Based on the lack of a distributed population of older stars, the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of 20 within the past ∼ 5 Myr. The optically bright stars in Serpens Northeast are visible because their natal molecular cloud has been eroded and not because they were flung outwards from a central factory of star formation. The separation between subclusters of 20-100 pc and the absence of an older population leads to speculation that an external forcing was needed to trigger the active star formation.

Local Molecular Gas toward the Aquila Rift Region

“…All of these show that molecular gas toward the Aquila Rift region is distributed in different distances, which agrees well with the new results from the very recent study (see details in Herczeg et al 2019). As shown in Figure 7 of Herczeg et al (2019), the ∼ 350-480 pc for cyan circles matches our value of ∼ 474 pc for the molecular gas toward the W40b region (the middle panel in Figure 5) and the > ∼ 550 pc for red circles agrees with the ∼ 560-670 pc toward the GMC complex G036.0+01.0 ( Figure 7).…”

“…All of these show that molecular gas toward the Aquila Rift region is distributed in different distances, which agrees well with the new results from the very recent study (see details in Herczeg et al 2019). As shown in Figure 7 of Herczeg et al (2019), the ∼ 350-480 pc for cyan circles matches our value of ∼ 474 pc for the molecular gas toward the W40b region (the middle panel in Figure 5) and the > ∼ 550 pc for red circles agrees with the ∼ 560-670 pc toward the GMC complex G036.0+01.0 ( Figure 7). The distance of ∼ 235 pc for the molecular gas at the W40a region (the left panel in Figure 5) is also consistent with the value of ∼ 250 pc for the extended dust cloud (i.e., the Serpens Cirrus, see Table 3 and the yellow contour in Figure 7 of Herczeg et al 2019).…”

“…All of these new results, together with other similar studies (e.g., see Table 3 and Figure 7 in Herczeg et al 2019), remarkably refine our knowledge on the local MCs. More recently, a narrow and elongated arrangement of dense gas in the local ISM is well unveiled based on the accurate distances of star forming regions toward the anticenter of the Milky Way (Alves et al 2020).…”

“…• 74, b = 3. • 12) (Herczeg et al 2019), and the LBN 031.75+04.98 (see red circles in Figure 2). This result is consistent with the work of Wang et al (2019) for the Gemini OB1 MCs, where the authors show that the high-temperature CO gas is related to nearby massive star-forming regions.…”

“…The velocity field of the dense molecular gas is also well described by the first moment of the 13 CO emission. Moreover, the concentration of the C 18 O gas represents the local dense gas region, where star formation activities are ongoing (i.e., many HH objects in LDN 673 discovered by Rector et al 2018; thousands of young stars and protostars in Serpens MC, W40 region, and Sh 2-62 studied by Herczeg et al 2019).…”