Search for: All records

Context.Acetone (CH₃COCH₃) is one of the most abundant three-carbon oxygen-bearing complex organic molecules (O-COMs) that have been detected in space. The previous detections were made in the gas phase toward star-forming regions that are chemically rich, mostly in protostellar systems. Recently, acetone ice has also been reported as (tentatively) detected toward two low-mass protostars, allowing comparisons in acetone abundances between gas and ice. The detection of acetone ice warrants a more systematic study of its gaseous abundances which is currently lacking. Aims.We aim to measure the gas-phase abundances of acetone in a large sample obtained from the CoCCoA program, and investigate the chemical evolution of acetone from ice to gas in protostellar systems. Methods.We fit the ALMA spectra to determine the column density, excitation temperature, and line width of acetone in 12 high-mass protostars as part of CoCCoA. We also constrained the physical properties of propanal (C₂H₅CHO), ketene (CH₂CO), and propyne (CH₃CCH), which might be chemically linked with acetone. We discuss the possible formation pathways of acetone by making comparisons in its abundances between gas and ice and between observations and simulations. Results.We firmly detect acetone, ketene, and propyne in the 12 high-mass protostars. The observed gas-phase abundances of acetone are surprisingly high compared to those of two-carbon O-COMs (especially aldehydes). Propanal is considered as tentatively detected due to lack of unblended lines covered in our data. The derived physical properties suggest that acetone, propanal, and ketene have the same origin from hot cores as other O-COMs, while propyne tends to trace the more extended outflows. The acetone-to-methanol ratios are higher in the solid phase than in the gas phase by one order of magnitude, which suggests gas-phase reprocessing after sublimation. There are several suggested formation pathways of acetone (in both ice and gas) from acetaldehyde, ketene, and propylene. The observed ratios between acetone and these three species are rather constant across the sample, and can be well reproduced by astrochemical simulations. Conclusions.On the one hand, the observed high gas-phase abundances of acetone along with dimethyl ether (CH₃OCH₃) and methyl formate (CH₃OCHO) may hint at specific chemical mechanisms that favor the production of ethers, esters, and ketones over alcohols and aldehydes. On the other hand, the overall low gas-phase abundances of aldehydes may result from destruction pathways that are overlooked or underestimated in previous studies. The discussed formation pathways of acetone from acetaldehyde, ketene, and propylene seem plausible from observations and simulations, but more investigations are needed to draw more solid conclusions. We emphasize the importance of studying acetone, which is an abundant COM that deserves more attention in the future. 

Context.Complex organic molecules (COMs) have been detected ubiquitously in protostellar systems. However, at shorter wavelengths (~0.8 mm), it is generally more difficult to detect larger molecules than at longer wavelengths (~3 mm) because of the increase in millimeter dust opacity, line confusion, and unfavorable partition function. Aims.We aim to search for large molecules (more than eight atoms) in the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 spectrum of IRAS 16293-2422 B. In particular, the goal is to quantify the usability of ALMA Band 3 for molecular line surveys in comparison to similar studies at shorter wavelengths. Methods.We used deep ALMA Band 3 observations of IRAS 16293-2422 B to search for more than 70 molecules and identified as many lines as possible in the spectrum. The spectral settings were set to specifically target three-carbon species such as i- and n-propanol and glycerol, the next step after glycolaldehyde and ethylene glycol in the hydrogenation of CO. We then derived the column densities and excitation temperatures of the detected species and compared the ratios with respect to methanol between Band 3 (~3 mm) and Band 7 (~1 mm, Protostellar Interferometric Line Survey) observations of this source to examine the effect of the dust optical depth. Results.We identified lines of 31 molecules including many oxygen-bearing COMs such as CH₃OH, CH₂OHCHO, CH₃CH₂OH, and c-C₂H₄O and a few nitrogen- and sulfur-bearing ones such as HOCH₂CN and CH₃SH. The largest detected molecules are gGg-(CH₂OH)₂and CH₃COCH₃. We did not detect glycerol or i- and n-propanol, but we do provide upper limits for them which are in line with previous laboratory and observational studies. The line density in Band 3 is only ~2.5 times lower in frequency space than in Band 7. From the detected lines in Band 3 at a ≳ 6σ level, ~25–30% of them could not be identified indicating the need for more laboratory data of rotational spectra. We find similar column densities and column density ratios of COMs (within a factor ~2) between Band 3 and Band 7. Conclusions.The effect of the dust optical depth for IRAS 16293-2422 B at an off-source location on column densities and column density ratios is minimal. Moreover, for warm protostars, long wavelength spectra (~3 mm) are not only crowded and complex, but they also take significantly longer integration times than shorter wavelength observations (~0.8 mm) to reach the same sensitivity limit. The 3 mm search has not yet resulted in the detection of larger and more complex molecules in warm sources. A full deep ALMA Band 2–3 (i.e., ~3–4 mm wavelengths) survey is needed to assess whether low frequency data have the potential to reveal more complex molecules in warm sources. 

Context. A low-mass companion potentially in the brown dwarf mass regime was discovered on a ~12 yr orbit (~5.5 au) around HD 167665 using radial velocity (RV) monitoring. Joint RV–astrometry analyses confirmed that HD 167665B is a brown dwarf with precisions on the measured mass of ~4–9%. Brown dwarf companions with measured mass and luminosity are valuable for testing formation and evolutionary models. However, its atmospheric properties and luminosity are still unconstrained, preventing detailed tests of evolutionary models. Aims. We further characterize the HD 167665 system by measuring the luminosity and refining the mass of its companion and reassessing the stellar age. Methods. We present new high-contrast imaging data of the star and of its close-in environment from SPHERE and GRAVITY, which we combined with RV data from CORALIE and HIRES and astrometry from HIPPARCOSandGaia. Results. The analysis of the host star properties indicates an age of 6.20 ± 1.13 Gyr. GRAVITY reveals a point source near the position predicted from a joint fit of RV data and HIPPARCOS–Gaiaproper motion anomalies. Subsequent SPHERE imaging confirms the detection and reveals a faint point source of contrast of ∆H2= 10.95 ± 0.33 mag at a projected angular separation of ~180 mas. A joint fit of the high-contrast imaging, RV, and HIPPARCOSintermediate astrometric data together with theGaiaastrometric parameters constrains the mass of HD 167665B to ~1.2%, 60.3 ± 0.7M_J. The SPHERE colors and spectrum point to an early or mid-T brown dwarf of spectral type T4₋₂⁺¹. Fitting the SPHERE spectrophotometry and GRAVITY spectrum with synthetic spectra suggests an effective temperature of ~1000–1150 K, a surface gravity of ~5.0–5.4 dex, and a bolometric luminosity log(L/L_⊙)=−4.892_−0.028^+0.024dex. The mass, luminosity, and age of the companion can only be reproduced within 3σby the hybrid cloudy evolutionary models of Saumon & Marley (2008, ApJ, 689, 1327), whereas cloudless evolutionary models underpredict its luminosity. 

Context.Since 2019, GRAVITY has provided direct observations of giant planets and brown dwarfs at separations of down to 95 mas from the host star. Some of these observations have provided the first direct confirmation of companions previously detected by indirect techniques (astrometry and radial velocities). Aims.We want to improve the observing strategy and data reduction in order to lower the inner working angle of GRAVITY in dual-field on-axis mode. We also want to determine the current limitations of the instrument when observing faint companions with separations in the 30–150 mas range. Methods.To improve the inner working angle, we propose a fiber off-pointing strategy during the observations to maximize the ratio of companion-light-to-star-light coupling in the science fiber. We also tested a lower-order model for speckles to decouple the companion light from the star light. We then evaluated the detection limits of GRAVITY using planet injection and retrieval in representative archival data. We compare our results to theoretical expectations. Results.We validate our observing and data-reduction strategy with on-sky observations; first in the context of brown dwarf follow-up on the auxiliary telescopes with HD 984 B, and second with the first confirmation of a substellar candidate around the starGaiaDR3 2728129004119806464. With synthetic companion injection, we demonstrate that the instrument can detect companions down to a contrast of 8 × 10⁻⁴(ΔΚ= 7.7 mag) at a separation of 35 mas, and a contrast of 3 × 10⁻⁵(ΔΚ= 11 mag) at 100 mas from a bright primary (K< 6.5), for 30 min exposure time. Conclusions.With its inner working angle and astrometric precision, GRAVITY has a unique reach in direct observation parameter space. This study demonstrates the promising synergies between GRAVITY andGaiafor the confirmation and characterization of substellar companions.