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Discovery of carbon monoxide emission from five debris disks around young A-type stars

A. Moór, P. Ábrahám, Á, Kóspál, G. Cataldi, A. M. Hughes, S. Marino, Q. Kral, J. Milli, N. Pawellek

TL;DR

This study expands the sample of CO-bearing debris disks around young, intermediate‑mass (primarily A‑type) stars by presenting ALMA Band 6 observations of 12 dust‑rich targets. CO gas is detected in five disks, including three CO‑rich systems with $M_ ext{CO}>10^{-3}$ $M_igoplus$, and the detection rate is significantly higher for hosts with $6.5~L_igodot<L_*ig<21.9~L_igodot$. By combining their results with literature, the authors show that CO gas in debris disks can arise from secondary production with shielding or from residual primordial gas, and that the gas content does not correlate simply with dust mass loss rates. The spatially resolved data reveal diverse gas/dust configurations, including co‑located belts, interior gas components, and multi‑ring structures, implying a range of dynamical histories and potential planetary sculpting. Overall, the work underscores the importance of stellar luminosity, shielding mechanisms, and disk stirring in determining the presence and evolution of gas in young debris disks and informs theories of planetary system development and atmospheric evolution.

Abstract

Over the past fifteen years, surveys mainly at millimeter wavelengths have led to the discovery of $\sim$20 gas-bearing debris disks, most of them surrounding young intermediate-mass stars. Exploring the properties and origin of this gas could be fundamental to better understanding the transition between the protoplanetary and debris disk phases, the evolution of icy planetesimal belts, and the formation of planetary atmospheres. To expand the list of known CO-bearing debris disks and to improve our knowledge of the environmental conditions under which they can form, we targeted twelve dust-rich debris disks around young ($<$50 Myr) intermediate-mass stars. Using the ALMA 12-m Array we performed millimeter continuum and CO line observations to search for dust and gas and to measure their quantity and spatial distribution. We discovered CO gas in five disks. Two of them have a low CO content of a few times 10$^{-5}$ M$_\oplus$, similar to that of $β$ Pic. The other three disks, however, are CO-rich with $M_\mathrm{CO}>10^{-3}$ M$_\oplus$. By combining our results with those of other studies we concluded, in agreement with previous findings, that the detection rate of CO gas is significantly higher for disks around stars with $6.5~L_\odot<L_*<21.9~L_\odot$ ($\sim$A8$-$A0 spectral type) than for disks around less luminous stars ($0.18~L_\odot<L_*<6.4~L_\odot$, K7$-$A9). A comparison of the measured CO masses and the estimated mass loss rates of solids in disks with low CO content ($<$10$^{-4}$ M$_\oplus$) suggests that collisions may play a role in CO gas production in such systems. Interestingly, however, the estimated mass loss rates of CO-rich debris disks are not higher than those of systems with low CO content. In light of this finding, we speculate on what could lead to the formation of CO-rich debris disks.

Discovery of carbon monoxide emission from five debris disks around young A-type stars

TL;DR

This study expands the sample of CO-bearing debris disks around young, intermediate‑mass (primarily A‑type) stars by presenting ALMA Band 6 observations of 12 dust‑rich targets. CO gas is detected in five disks, including three CO‑rich systems with , and the detection rate is significantly higher for hosts with . By combining their results with literature, the authors show that CO gas in debris disks can arise from secondary production with shielding or from residual primordial gas, and that the gas content does not correlate simply with dust mass loss rates. The spatially resolved data reveal diverse gas/dust configurations, including co‑located belts, interior gas components, and multi‑ring structures, implying a range of dynamical histories and potential planetary sculpting. Overall, the work underscores the importance of stellar luminosity, shielding mechanisms, and disk stirring in determining the presence and evolution of gas in young debris disks and informs theories of planetary system development and atmospheric evolution.

Abstract

Over the past fifteen years, surveys mainly at millimeter wavelengths have led to the discovery of 20 gas-bearing debris disks, most of them surrounding young intermediate-mass stars. Exploring the properties and origin of this gas could be fundamental to better understanding the transition between the protoplanetary and debris disk phases, the evolution of icy planetesimal belts, and the formation of planetary atmospheres. To expand the list of known CO-bearing debris disks and to improve our knowledge of the environmental conditions under which they can form, we targeted twelve dust-rich debris disks around young (50 Myr) intermediate-mass stars. Using the ALMA 12-m Array we performed millimeter continuum and CO line observations to search for dust and gas and to measure their quantity and spatial distribution. We discovered CO gas in five disks. Two of them have a low CO content of a few times 10 M, similar to that of Pic. The other three disks, however, are CO-rich with M. By combining our results with those of other studies we concluded, in agreement with previous findings, that the detection rate of CO gas is significantly higher for disks around stars with (A8A0 spectral type) than for disks around less luminous stars (, K7A9). A comparison of the measured CO masses and the estimated mass loss rates of solids in disks with low CO content (10 M) suggests that collisions may play a role in CO gas production in such systems. Interestingly, however, the estimated mass loss rates of CO-rich debris disks are not higher than those of systems with low CO content. In light of this finding, we speculate on what could lead to the formation of CO-rich debris disks.

Paper Structure

This paper contains 31 sections, 4 equations, 13 figures, 8 tables.

Table of Contents

  1. Introduction
  2. Sample selection
  3. Observations and data reduction
  4. Analysis of continuum data
  5. Imaging
  6. Modeling of the continuum emission
  7. Dust masses
  8. Analysis of line data
  9. Imaging and CO line fluxes
  10. CO mass estimates
  11. Spatial distribution of CO gas
  12. Discussion
  13. Sample of young dust-rich debris disks observed by the ALMA
  14. Detection rate of CO gas
  15. Radial location of planetesimal belts in CO bearing debris systems
  16. ...and 16 more sections

Figures (13)

  • Figure 1: ALMA Band 6 continuum emission for the observed targets. The stellar position is marked with a black plus sign. In the case of HD 31305, the position of the primary component (HD 31305 A) is shown by a white plus sign, while the companion star (HD 31305 B) is marked by a black diamond. For HD 131960 the visibility data are tapered by a Gaussian with an FWHM of 1. At the bottom left of each panel a filled white ellipse shows the beam size. The length of the horizontal black bars corresponds to 100 au. The color bar units are mJy beam$^{-1}$.
  • Figure 1: Left and middle panels: $^{12}$CO and $^{13}$CO J=2--1 moment 0 maps for HD 31305 B, respectively. At the bottom left of each panel a filled white ellipse shows the beam size. The length of the horizontal white bars correspond to 100 au. The white plus sign shows the position of HD 31305 A while the white diamond displays the position of HD 31305 B. The color bar units are mJy beam$^{-1}$ km s$^{-1}$. The right panel displays the obtained CO spectra in all three isotopologs. For clarity, the $^{13}$CO and C$^{18}$O spectra have been shifted downward. The horizontal dashed lines show the zero flux levels of the spectra.
  • Figure 2: $^{12}$CO (2--1) and $^{13}$CO (2--1) moment 0 maps for HD 9985, HD 145101, HD 155853, and HD 152989 (left and middle columns). At the bottom left of each panel a filled white ellipse shows the beam size. The length of the horizontal black bars corresponds to 100 au. The color bar units are mJy beam$^{-1}$ km s$^{-1}$. The contours plotted over the moment 0 maps of HD 9985 and HD 155853 show the continuum emission. The contour levels are in steps of [5,8,11,14,17]$\times$ rms noise of 15.9 $\mu$Jy beam$^{-1}$ for HD 9985 and [4,6,8,10]$\times$ rms noise of 15.3 $\mu$Jy beam$^{-1}$ for HD 155853. The right column displays the obtained CO spectra in all three isotopologs. For better visibility, the $^{13}$CO and C$^{18}$O spectra have been shifted downward, and in the case of HD 9985 and HD 155853 the C$^{18}$O spectrum is multiplied by 4. The horizontal dashed lines show the zero flux levels of the spectra. The vertical dash-dotted and dotted lines mark the radial velocity of the star and its uncertainty in the LSR frame.
  • Figure 3: Left panel: Integrated $^{12}$CO (2--1) intensity map of HD 170116 between $v_\mathrm{LSR}$ of $-$4.4 and $+$2.8 km s$^{-1}$, using natural weighted data. The applied velocity interval was calculated under the assumption that the possible gas material is co-located with the detected dust (Sect. \ref{['sec:coimaging']}). The color bar units are mJy beam$^{-1}$ km s$^{-1}$. The moment 0 map is overlaid by the contours of the natural-weighted continuum image of the source. The contour levels are at 3, 6, 9, 12, and 13$\times$ rms of the continuum image. The white ellipse in the lower left corner shows the synthetic beam. The size of the horizontal black bar in the upper left corner corresponds to 100 au. The black plus sign indicates the position of the star. Center panel: same as the left panel, but zoomed in to the center of the observed localized CO brightness peak and using a narrower integration velocity range from $+$0.4 and $+$2.8 km s$^{-1}$, corresponding to the interval in which the emission is detected. Right panel: spectrum of the observed CO peak. The vertical dash-dotted and dotted lines mark the radial velocity of the star and its uncertainty in the LSR frame.
  • Figure 4: a--d): Position-velocity diagrams of $^{12}$CO (2-1) emission for HD 9985 (a), HD 155853 (b), HD 152989 (c), and HD 170116 (d). The color bar units are mJy beam$^{-1}$ channel$^{-1}$. The black solid curves show the tangential velocity of the gas, while the black dashed diagonal lines display the line-of-sight velocity of the gas at a fixed orbital radius as a function of projected separation. To derive these, we assume a Keplerian rotation profile for the gas, the distance and mass of the star are taken from from Table \ref{['tab:targets']}, and the position angle and inclination of the gas disk are from Sect. \ref{['sec:cospatial']}. The white rectangle at the bottom left represents the spectro-spatial resolution. e): Azimuthally averaged radial profile for the continuum and $^{12}$CO (2-1) data for HD 9985 (top) and HD 155853 (bottom). For comparison, the profiles are normalized to unity.
  • ...and 8 more figures