Hunting pre-stellar cores with APEX: overview

TL;DR

This study outlines an unbiased program to identify nearby pre-stellar cores by combining Herschel-based density criteria with high-density tracer spectroscopy from APEX. By selecting cores with central densities $n_{ m H_2} \,\ge\,3\times10^{5}$ cm$^{-3}$ and observing $ m N_2H^+$ (3-2) and $ m N_2D^+$ (4-3), the authors identify 17 bona fide pre-stellar cores ($R_D>0.10$) among 33 pre-stellar targets, along with 16 dynamically evolved starless cores and 7 YSO-associated objects. The results demonstrate that dust continuum emission plus high-density molecular lines robustly identifies pre-stellar cores and reveal a range of deuteration levels, informing on the early chemical and physical structure. The work lays a statistically significant foundation for follow-up interferometric studies (e.g., ALMA) to resolve the central kernel, trace temperature and density profiles, and study chemical inheritance from clouds to disks and planets.

Abstract

[Abridged] $Context.$ Pre-stellar cores are centrally concentrated starless cores on the verge of star formation and they represent the initial conditions for star and planet formation. Pre-stellar cores host an active organic chemistry and isotopic fractionation, kept stored into thick icy mantles, which can be inherited by the future protoplanetary disks and planetesimals. So far, only a few have been studied in detail, with special attention being paid to L1544 in the Taurus Molecular Cloud. $Aims.$ The aim is to identify nearby ($<$200 pc) pre-stellar cores in an unbiased way, to build a sample that can then be studied in detail. $Methods.$ We first used the Herschel Gould Belt Survey archival data, selecting all those starless cores with central H$_2$ number densities higher than or equal to 3$\times$10$^5$ cm$^{-3}$, the density of L1544 within the Herschel beam. The selected 40 (out of 1746) cores have then been observed in N$_2$H$^+$(3-2) and N$_2$D$^+$(4-3) using the APEX antenna. $Results.$ A total of 17 bona-fide (i.e., with a deuterium fraction larger than 10%) pre-stellar cores have been identified. Other 16 objects can also be considered pre-stellar, as they are dynamically evolved starless cores, but their deuterium fraction is relatively low ($<$10%). The remaining 7 objects have been found associated with very young stellar objects. $Conclusions.$ Dust continuum emission, together with spectroscopic observations of N$_2$H$^+$(3-2) and N$_2$D$^+$(4-3), is a powerful tool to identify pre-stellar cores in molecular clouds. Detailed modeling of the physical structure of the objects is now required for reconstructing the chemical composition as a function of radius. This work has provided a statistically significant sample of 33 pre-stellar cores, a crucial step in the understanding of the process of star and planet formation.

Figures (7)

• Figure 1: Spectra of $\rm N_2H^+$ (3-2) toward the whole sample of pre-stellar core candidates in Table \ref{['Tab:targets']}. From top to bottom and then from left to right, the spectra are ordered in integrated intensity, with CRA 047 being the strongest. The spectra have also been displaced in intensity by multiples of 2 K and centered at 0 LSR velocity, to allow comparison. Note that the spectra of the first 10 objects have been divided by factors between 6 and 2 (see labels) for clarity. Asterisks next to the names indicate the association with a young stellar object (YSO).
• Figure 2: Spectra of $\rm N_2D^+$ (4-3) toward the whole sample of pre-stellar core candidates in Table \ref{['Tab:targets']}. The spectra follow the same order as in Fig. \ref{['Fig:n2hp32']}. The spectra have also been displaced in intensity by multiples of 2 K and centered at 0 LSR velocity, to allow comparison. Note that the spectrum of Oph 464 has been divided by a factor of 3 (see label) for clarity.
• Figure 3: Spectra of $\rm N_2D^+$(3-2), $\rm N_2H^+$(5-4), and $\rm N_2D^+$(6-5) toward a sub-sample of 23 pre-stellar core candidates. From top to bottom, the spectra follow the same order as in Fig. \ref{['Fig:n2hp32']}. The spectra have also been displaced in intensity by 2 K and centered at 0 LSR velocity, to allow comparison. Note that some spectra have been divided or multiplied by factors of 2 to 5 (see labels) for clarity.
• Figure 4: Comparison of centroid velocities ($\rm v_{LSR}$) and line widths (FWHM) of the $\rm N_2D^+$(4-3) and $\rm N_2H^+$(3-2) lines. The different colors represent different molecular cloud complexes: Corona Australis (blue), Ophiuchus (red), Lupus (black). The stars show the cores associated with young stellar objects. The 1:1 correlation line is shown in dashed black.
• Figure 5: $\rm N_2H^+$ deuterium fraction ($R_{\rm D} \equiv$ N($\rm N_2D^+$)/N($\rm N_2H^+$) column density ratio) obtained from the $\rm N_2D^+$(4-3) and $\rm N_2H^+$(3-2) lines using constant excitation temperature analysis, as a function of H$_2$ column density, N(H$_2$), from Herschel data. The different colors represent different molecular cloud complexes: Corona Australis (blue), Ophiuchus (red), Lupus (black). The stars show the cores associated with young stellar objects.