Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Extended and Compact Ortho-H$_2$D$^+$ Structures Close to the Moment of Star-Formation: Evidence from ALMA-ACA Observations in Taurus

Kazuki Tokuda, Kenji Furuya, Naofumi Fukaya, Kengo Tachihara, Ken'ichi Tatematsu, Shingo Nozaki, Nami Sakai, Yuri Aikawa, Kazuya Saigo, Toshikazu Onishi, Masahiro N. Machida

TL;DR

This study uses ALMA-ACA stand-alone observations to map ortho-$H_2D^+$ in three Taurus cores spanning prestellar to early protostellar stages, aiming to trace core-center evolution over $10^4$ years. It finds that $H_2D^+$ is predominantly extended, detected by single-dish total-power data across all targets, while interferometric 7 m data reveal compact emission only toward MC35-mm, with upper-limit abundances of about $2\times10^{-11}$ for L1544 and $7\times10^{-12}$ for MC27, and $\sim3\times10^{-11}$ for MC35-mm. The results imply rapid depletion of $H_2D^+$ in dense, central 1,000 au regions due to conversion to more deuterated species, though a mild temperature rise in the first-core phase can enhance brightness sufficiently for detection. Overall, $H_2D^+$ traces extended, cold gas in the early stages and remains a potential tracer for identifying cores near the onset of star formation, provided spatial filtering and chemical evolution are carefully considered. These findings motivate complementary studies of related deuterated species and detailed chemical-kinematic modeling to interpret core-nucleus structure across evolutionary stages.

Abstract

Observing and characterizing pre- and protostellar cores in the earliest and densest stages of star formation is challenging due to their short timescales and high densities, limiting the suitable tracers and targets. We conducted ALMA-Atacama Compact Array (ACA) stand-alone observations of ortho-H$_2$D$^+$ (1$_{\rm 1,0}$-1$_{\rm 1,1}$) emission, which is believed to trace cold high-density regions, toward three dense cores in the Taurus molecular cloud: (1) L1544, likely in the densest prestellar phase; (2) MC 35-mm, a candidate for the first hydrostatic core; and (3) MC 27/L1521F, which hosts a Class 0 very-low luminosity object. These observations provide high angular resolution data for the line across a set of cores selected to represent consecutive stages around the onset of star formation, offering a unique opportunity to trace the time evolution of $\sim$10$^4$ years. With the single-dish total-power array, we detected ortho-H$_2$D$^+$ emission in all three cores, revealing its presence over scales of $\sim$10,000 au. In the interferometric 7 m array data with a beam size of 3.$''$5 ($\sim$500\,au), emission was detected only toward the central continuum source of MC 35-mm, with a significance of $\sim$3$σ$. No significant detections were found in the other targets, placing an upper limit on the H$_2$D$^{+}$ abundance of $\sim$10$^{-11}$ in the dense components traced by the interferometric continuum emission. These results suggest that ortho-H$_2$D$^+$ predominantly exhibits an extended distribution over several thousand au in the early stages of star formation. Detection in compact, dense central structures may only be achieved within a few $\times$ 10$^{4}$ years immediately before or after protostar formation.

Extended and Compact Ortho-H$_2$D$^+$ Structures Close to the Moment of Star-Formation: Evidence from ALMA-ACA Observations in Taurus

TL;DR

This study uses ALMA-ACA stand-alone observations to map ortho- in three Taurus cores spanning prestellar to early protostellar stages, aiming to trace core-center evolution over years. It finds that is predominantly extended, detected by single-dish total-power data across all targets, while interferometric 7 m data reveal compact emission only toward MC35-mm, with upper-limit abundances of about for L1544 and for MC27, and for MC35-mm. The results imply rapid depletion of in dense, central 1,000 au regions due to conversion to more deuterated species, though a mild temperature rise in the first-core phase can enhance brightness sufficiently for detection. Overall, traces extended, cold gas in the early stages and remains a potential tracer for identifying cores near the onset of star formation, provided spatial filtering and chemical evolution are carefully considered. These findings motivate complementary studies of related deuterated species and detailed chemical-kinematic modeling to interpret core-nucleus structure across evolutionary stages.

Abstract

Observing and characterizing pre- and protostellar cores in the earliest and densest stages of star formation is challenging due to their short timescales and high densities, limiting the suitable tracers and targets. We conducted ALMA-Atacama Compact Array (ACA) stand-alone observations of ortho-HD (1-1) emission, which is believed to trace cold high-density regions, toward three dense cores in the Taurus molecular cloud: (1) L1544, likely in the densest prestellar phase; (2) MC 35-mm, a candidate for the first hydrostatic core; and (3) MC 27/L1521F, which hosts a Class 0 very-low luminosity object. These observations provide high angular resolution data for the line across a set of cores selected to represent consecutive stages around the onset of star formation, offering a unique opportunity to trace the time evolution of 10 years. With the single-dish total-power array, we detected ortho-HD emission in all three cores, revealing its presence over scales of 10,000 au. In the interferometric 7 m array data with a beam size of 3.5 (500\,au), emission was detected only toward the central continuum source of MC 35-mm, with a significance of 3. No significant detections were found in the other targets, placing an upper limit on the HD abundance of 10 in the dense components traced by the interferometric continuum emission. These results suggest that ortho-HD predominantly exhibits an extended distribution over several thousand au in the early stages of star formation. Detection in compact, dense central structures may only be achieved within a few 10 years immediately before or after protostar formation.

Paper Structure

This paper contains 5 sections, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Target Descriptions, ALMA Observations, and Data Reductions
  3. Results
  4. Discussion
  5. Concluding remarks

Figures (4)

  • Figure 1: Spatial distribution and spectra of the 0.83 mm continuum and ortho-H$_2$D$^+$(1$_{\rm 1,0}$--1$_{\rm 1,1}$) in L1544. (a) The color-scale image shows the 0.83 mm continuum obtained with the 7 m array. The contours represent the continuum emission at levels of 3, 5, and 7$\sigma$ (1$\sigma$ = 0.78 mJy beam$^{-1}$). The synthesized beam size is shown in the bottom-left corner. (b) The color-scale image shows the integrated intensity of H$_2$D$^+$ over the velocity range of 7.0--7.5 km s$^{-1}$ obtained with the 7 m array. The contours and the ellipse in the bottom-left corner are the same as those in panel (a). (c) The color scale shows the integrated intensity of H$_2$D$^+$ over the velocity range of 6.8--7.8 km s$^{-1}$ obtained with the TP array. The beam size of the TP array, 17$.\!\!^{\prime\prime}$5, is shown in the bottom-left corner. The contours are the same as those in panel (a). (d) The yellow, magenta, and blue spectra represent the H$_2$D$^+$ spectra obtained with the 7 m array, the 7 m array averaged over a region corresponding to the TP beam size, and the TP array, respectively. The extraction regions are indicated by dashed circles in the corresponding colors in panels (b) and (c).
  • Figure 2: Same as those in Figure \ref{['fig:L1544']} but for MC 35-mm. The velocity ranges integrated for the H$_2$D$^+$ data cube are 5.8--5.9 km s$^{-1}$ in panel (b) and 5.5--6.5 km s$^{-1}$ in panel (c). In panel (d), the spectrum from the 7 m array (orange) was extracted by averaging over a smaller region centered on the emission peak in panel (b), in contrast to the broader areas used for the other two sources.
  • Figure 3: Same as those in Figure \ref{['fig:L1544']} but for MC 27. The velocity ranges integrated for the H$_2$D$^+$ data cube are 6.0--7.0 km s$^{-1}$ in panel (b) and (c).
  • Figure 4: (a) H$_2$ density and (b) temperature profiles adapted from Masunaga_2000. The $x$-axis represents the radial offset in arcseconds assuming the typical distance to the Taurus molecular cloud ($D$$\sim$140 pc). (c) Radial intensity (peak brightness temperature) profiles of ortho-H$_2$D$^+$ derived from the first-core and prestellar core models. The green dotted line shows half of the beam size, 2$\hbox{$^{\prime\prime}$}$, with the 7 m array. (d) Radial profiles of the fractional abundance of ortho-H$_2$D$^+$ relative to H$_2$ for the first-core and prestellar core models. (e) The normalized radial intensity profile of ortho-H$_2$D$^+$ and $N_{\rm H_2}$ from the prestellar core model.