FAUST XXIX. OCS line emission: a new method for measuring the luminosity of embedded protostars in binary systems

Abstract

The luminosity of embedded protostars is commonly measured via observations of the dust continuum spectral energy distribution from millimetre to infrared wavelengths. However, this method cannot be applied to embedded protostars in binary or multiple systems, where their components are usually unresolved over this extended wavelength range. We propose a new method, based on the idea that a molecule formed (mainly) on the grain surfaces only emits lines in the region where it thermally sublimates from the grain mantles, heated by the photons emitted by the embedded source. In this respect, carbonyl sulfide (OCS) is an optimal molecule, because of its low binding energy and rotational lines in the millimetre. We apply the method to the protobinary system NGC1333 IRAS4A, using ALMA high-spatial resolution ($\sim$50 au) observations of the OCS(19-18) line as part of the ALMA Large Program FAUST. We also present new quantum mechanics calculations of the OCS binding energy distribution, essential for the application of the method. We found that the two binary components, A1 and A2, have a comparable luminosity within the error bars, 7.5$\pm$2.5 and 7$\pm$1 L$_\odot$, respectively. We discuss the reliability of the estimated luminosities and the potential of this new method for measuring the luminosity of embedded protostars in binary and multiple systems.

Figures (10)

• Figure 1: BSSE corrected BE distribution of OCS on a model icy cluster at DLPNO-CCSD(T) level. Structures and ZPEs are calculated at ONIOM(B97-3c:GFN2-XTB) level. The dashed blue curve is the f$_\mathrm{Gauss}$(hist(BE), $\sigma$, $\mu$) not normalized Gaussian best fit function for the histogram.
• Figure 2: Left panel: Moment 0 of OCS(19--18) integrated between $-$0.15 km/s and +14.64 km/s, where the sensibility is optimised. Contours start at 5$\sigma$ ($\sigma$ = 3.03 mJy/beam km/s) and increase by steps of 20$\sigma$. Both position of the two protostars A1 and A2, is represented by white stars. Note that for A2 the position is taken to be the peak of OCS emission (see text). Red and blue arrows represent the red-shifted and blue-shifted outflow directions, respectively Chahine2024-cav. The synthesised beam is the yellow ellipse in the bottom right corner of the map. Middle and right panels: Moment 0 maps of OCS(19--18) integrated over different velocity intervals: $\pm[1;1.5]$ km/s (LV) and $\pm[1.7;2.4]$ km/s (IV) with respect to V$\rm{_{lsr}}$ = 7 km/s. Contours start at 5$\sigma$ ($\sigma$ = 0.62 mJy/beam km/s and 0.70 mJy/beam km/s for LV and IV panels, respectively) and increase by steps of 10$\sigma$. Note that, in this case, we optimised the resolution (see text). Both position of the two protostars A1 and A2 are represented by black stars. Black lines represent the direction of the envelope where OCS is thermally sublimated and used for the model analysis (see text). The synthesised beam is the black ellipse in the bottom right corner of each map.
• Figure 3: Top left panel: Moment 0 of the OCS line intensity around A2. The intensity is integrated between +2.74 km/s and +14.17 km/s. Contours start at 5$\sigma$ ($\sigma$ = 3.5 mJy/beam km/s) and increase by steps of 20$\sigma$. The blue-shifted emission corresponds to the west side of the star, while the red-shifted emission to the east part (see Fig. \ref{['fig:ocs-mom0']}). The grey star marks the continuum emission peak while the orange star the OCS emission peak. The grey and orange lines show the PA = 120°, centred on the two emission peaks, respectively. The synthesised beam is represented in yellow in the bottom right corner of the map. Top centre and right panels:$\chi^2$ of the difference between blue- and red-shifted line intensity varying the PA between 115 and 125°, for the centre set on the OCS line (orange) and dust continuum (grey) emission peaks, respectively. Bottom panels: Normalized OCS line intensity as a function of the distance from the centre set on the OCS line (left panel) and dust continuum (right panel) emission peaks with PA equal to 120°. The red and blue crosses represent the OCS line intensity toward the red- and blue-shifted emission, respectively. The line intensity error bars, computed as 3$\sigma$ ($\sigma$ = 3.5 mJy/beam km/s), are the solid black lines, mostly masked by the crosses. The black dashed line shows the synthesised beam profile.
• Figure 4: Left panel: Normalised OCS gas abundance profile towards A2 for five luminosities: 2 (blue), 5 (orange), 7.5 (green), 10 (red) and 15 (purple) L$_\odot$. Right panel: Theoretical intensity profile depending on the luminosity value. The dashed line shows the synthesised beam profile of 0$\farcs$25.
• Figure 5: Top panels: Normalised $\chi^2$ as a function of the A2 luminosity, for PA=118° (left) and 123° (right), where the two innermost data points of the observations are not considered (see text). Bottom panels: Normalised line intensity profile towards A2. The blue dashed lines show the theoretical profile, normalised and beam-convolved, assuming an A2 luminosity of 6 and 8 L$_\odot$ for PA=118 and 123°, respectively. The crosses are the normalised average of the intensity observed in the red- and blue- shifted emitting regions with its error bars, which are given by the difference of the red- and blue- shifted emitting regions (added to the 3$\sigma$ rms). The dashed lines show the beam profile.