Molecular Distributions of the Protostellar Envelope and the Outflow of IRAS 15398–3359: Principal Component Analysis

Abstract

Using the Atacama Large Millimeter/submillimeter Array, we have imaged 15 molecular-line emissions and the dust continuum emission around the Class 0 protostellar source IRAS 15398−3359. The outflow structure is mainly traced by the H2CO (K a = 0 and 1), CCH, and CS emissions. These lines also trace the disk/envelope structure around the protostar. The H2CO (K a = 2 and 3), CH3OH, and SO emissions are concentrated toward the protostar, while the DCN emission is more extended around the protostar. We have performed principal component analysis (PCA) for these distributions on two different scales, the outflow and the disk/envelope structure. For the latter case, the molecular-line distributions are classified into two groups, according to the contribution of the second principal component, one having a compact distribution around the protostar and the other showing a rather extended distribution over the envelope. Moreover, the second principal component value tends to increase as an increasing quantum number of H2CO (K a = 0, 1, 2, and 3), reflecting the excitation condition: the distribution is more compact for higher excitation lines. These results indicate that PCA is effective at extracting the characteristic features of the molecular-line distributions around the protostar in an unbiased way. In addition, we identify four blobs in the outflow structure in the H2CO lines, some of which can also be seen in the CH3OH, CS, CCH, and SO emissions. The gas temperature derived from the H2CO lines ranges from 43–63 K, which suggests shocks due to the local impact of the outflow on clumps of the ambient gas.

FAUST. II. Discovery of a Secondary Outflow in IRAS 15398−3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?

“…The CH 3 OH and C 18 O line emission traces part of the outflow in Figures 2(b) and (c). The knot can also be seen in the CH 3 OH emission (see also Okoda et al 2020). Along with these previously known structures, we have found an additional spatial feature extending toward the southeast and northwest of IRAS 15398−3359.…”

“…220°) is seen in the H 2 CO emission, as reported previously (Oya et al 2014;Bjerkeli et al 2016b). Contrastingly, the SO emission is concentrated around the protostar (Okoda et al 2018(Okoda et al , 2020 with little emission within the outflow except for a localized knot (Blob D in Okoda et al 2020) seen in the southwestern lobe, which could be formed by the impact of the outflow on ambient gas. The CH 3 OH and C 18 O line emission traces part of the outflow in Figures 2(b) and (c).…”

“…For these reasons, the resolved-out effect should not affect the molecular abundances significantly. For the protostar position, the C 18 O column density is derived to be (5.5−6.4) × 10 15 cm −2 , where the gas temperature and the H 2 density are assumed to be 54 K (Okoda et al 2020) and 10 5 -10 7 cm −3 , respectively. Then, the abundance ratios relative to C 18 O are evaluated from the column densities, where the same temperature is assumed for the molecular species and C 18 O.…”

“…a The positions are shown in Figures 1 and 2. b The temperature and the H 2 density are assumed to be 54 K (Okoda et al 2020) and 10 5 -10 7 cm −3 , respectively. A range of column density is shown for each molecule.…”

Clustered star formation at early evolutionary stages

“…The emission of MP4 is elongated to the north-east toward mm core 6. All species having an enhanced emission toward MP4 and MP5 are known to trace shocked regions caused by protostellar outflows (Leurini et al 2011;Benedettini et al 2013;Moscadelli et al 2013;Shimajiri et al 2015;Palau et al 2017;Tychoniec et al 2019;Okoda et al 2020;Taquet et al 2020). There is enhanced c-C 3 H 2 emission toward the north and south of core 2 (Fig.…”