תובנה - Astronomy and Astrophysics - # Molecular gas composition in embedded low-mass protostars

Molecular Emission and Absorption in Low-Mass Protostars Observed with the James Webb Space Telescope

Q: How do the molecular abundances and excitation conditions in low-mass protostars compare to those in more evolved protoplanetary disks?

The study of molecular abundances and excitation conditions in low-mass protostars reveals significant differences when compared to more evolved protoplanetary disks. In low-mass protostars, the typical excitation temperatures of gas-phase molecules range from 100 to 300 K, which is considerably lower than the temperatures observed in Class II protoplanetary disks, where temperatures often exceed 500 K. This temperature disparity suggests that the physical conditions in low-mass protostars are less energetic, likely due to their earlier evolutionary stage and the presence of dense, cold material surrounding the protostar. Molecular abundances also differ notably. In low-mass protostars, water (H2O) is the most commonly detected molecule, followed by CO2, CO, and OH, with other molecules like C2H2 and HCN detected in fewer sources. In contrast, more evolved protoplanetary disks exhibit a richer diversity of molecular species, including complex organics and hydrocarbons, which are indicative of more advanced chemical processing and thermal conditions. The presence of these molecules in disks is often attributed to processes such as ice sublimation and gas-phase chemistry that occur at higher temperatures, facilitating the formation of more complex molecules.

Q: What are the implications of the observed differences in molecular emission/absorption properties between sources, and how do they relate to the physical and chemical evolution of the protostellar system?

The observed differences in molecular emission and absorption properties among various low-mass protostars have profound implications for understanding the physical and chemical evolution of protostellar systems. For instance, the detection of hot water (∼1000–1200 K) in some sources indicates the presence of hot molecular gas, likely originating from embedded disks or shock interactions, suggesting that these regions are undergoing significant thermal and dynamical processes. Moreover, the variations in molecular emission and absorption features can provide insights into the different evolutionary stages of protostellar systems. For example, sources exhibiting strong absorption features may indicate the presence of cooler, denser material, while those with prominent emission lines may suggest active outflows or disk winds. The differences in molecular ratios with respect to H2O between ice and gas phases also hint at processes such as ice sublimation and high-temperature gas-phase chemistry, which are critical for understanding the transition from protostellar to protoplanetary phases. These findings underscore the complexity of chemical evolution in protostellar systems, where the interplay between temperature, density, and molecular composition shapes the pathways of star and planet formation.

Q: Could the detection of unexpected molecules like SiO, CS, and NH3 at mid-IR wavelengths provide new insights into the chemical processes occurring in the inner regions of low-mass protostars?

The detection of unexpected molecules such as SiO, CS, and NH3 at mid-IR wavelengths indeed offers new insights into the chemical processes occurring in the inner regions of low-mass protostars. These molecules are typically associated with high-temperature environments and shock chemistry, suggesting that their presence indicates active physical processes such as outflows, shocks, or interactions between the protostar and its surrounding material. For instance, SiO is often linked to shock-induced chemistry, where it can be produced through the sputtering of silicate grains in the presence of high-energy events like outflows. The detection of SiO in low-mass protostars implies that these systems may experience significant shock activity, which can enhance the chemical complexity and lead to the formation of various molecular species. Similarly, the presence of CS and NH3 can provide clues about the thermal and chemical conditions in the inner regions of protostars. CS is a key molecule in the study of sulfur chemistry, while NH3 is crucial for understanding nitrogen chemistry in these environments. Their detection at mid-IR wavelengths suggests that the inner regions of low-mass protostars are not only chemically rich but also dynamically active, with ongoing processes that influence the overall evolution of the protostellar system. In summary, the detection of these unexpected molecules enhances our understanding of the intricate chemical networks and physical conditions present in the early stages of star formation, highlighting the importance of mid-IR observations in probing the molecular landscape of low-mass protostars.

מושגי ליבה

The JWST/MIRI-MRS observations reveal a remarkable richness in molecular features across the mid-infrared wavelength range, tracing various warm components in young protostellar systems, from the hot core regions to shocks in the outflows and disk winds. The typical temperatures of the gas-phase molecules of 100-300 K are consistent with both ice sublimation in hot cores as well as high-temperature gas phase chemistry.

תקציר

The JWST Observations of Young protoStars (JOYS) program has observed 18 low-mass protostars using the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope. This paper presents an overview of the gaseous molecular emission and absorption features detected in the spectra extracted from the central protostellar positions.

The key highlights and insights are:

Continuum emission is detected across the full MIRI-MRS wavelength range toward 16 out of 18 sources, with the other two being too embedded to be detected.
The MIRI-MRS spectra show a remarkable richness in molecular features, with water being the most commonly detected molecule (12 out of 16 sources), followed by CO2 (11 out of 16), CO (8 out of 16), and OH (7 out of 16). Other molecules such as 13CO2, C2H2, HCN, CH4, and SO2 are detected toward at most three sources.
The JOYS data also yield the surprising detection of SiO gas toward two sources and the first detection of CS and NH3 at mid-IR wavelengths toward a low-mass protostar.
The temperatures derived for the majority of the molecules are 100-300 K, much lower than what is typically derived toward more evolved Class II sources (≳500 K). This suggests that the mid-IR molecular features are not tracing the inner embedded disks.
Toward three sources, hot (∼1000-1200 K) H2O is detected, indicative of the presence of hot molecular gas in the embedded disks, but such warm emission from other molecules is absent.
The agreement in abundance ratios with respect to H2O between ice and gas point toward ice sublimation in a hot core for a few sources, whereas their disagreement and velocity offsets hint at high-temperature (shocked) conditions toward other sources.

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arxiv.org

סטטיסטיקה

The typical noise level (1σ) in the MIRI-MRS spectra is 0.1-0.5 mJy in channels 1-3, increasing up to ~10 mJy in channel 4C.
The radial velocity offsets (vline) with respect to the source vlsr were determined for CO2, SiO, and H2O in the three most line-rich sources (B1-c, L1448-mm, BHR71-IRS1) to be ~5 km/s. For other species and sources, the velocity offsets were determined by visual inspection with an uncertainty of ~10-20 km/s.

ציטוטים

"The MIRI-MRS spectra show a remarkable richness in molecular features across the full wavelength range, in particular toward B1-c (absorption) and L1448-mm (emission)."
"The temperatures derived for the majority of the molecules are 100-300 K, much lower than what is typically derived toward more evolved Class II sources (≳500 K)."
"The agreement in abundance ratios with respect to H2O between ice and gas point toward ice sublimation in a hot core for a few sources (e.g., B1-c) whereas their disagreement and velocity offsets hint at high-temperature (shocked) conditions toward other sources (e.g., L1448-mm, BHR71-IRS1)."

תובנות מפתח מזוקקות מ:

JWST Observations of Young protoStars (JOYS). Overview of gaseous molecular emission and absorption in low-mass protostars

by M. L... ב- arxiv.org 10-03-2024

https://arxiv.org/pdf/2410.01636.pdf

JWST Observations of Young protoStars (JOYS). Overview of gaseous molecular emission and absorption in low-mass protostars

שאלות מעמיקות

How do the molecular abundances and excitation conditions in low-mass protostars compare to those in more evolved protoplanetary disks?

The study of molecular abundances and excitation conditions in low-mass protostars reveals significant differences when compared to more evolved protoplanetary disks. In low-mass protostars, the typical excitation temperatures of gas-phase molecules range from 100 to 300 K, which is considerably lower than the temperatures observed in Class II protoplanetary disks, where temperatures often exceed 500 K. This temperature disparity suggests that the physical conditions in low-mass protostars are less energetic, likely due to their earlier evolutionary stage and the presence of dense, cold material surrounding the protostar.
Molecular abundances also differ notably. In low-mass protostars, water (H2O) is the most commonly detected molecule, followed by CO2, CO, and OH, with other molecules like C2H2 and HCN detected in fewer sources. In contrast, more evolved protoplanetary disks exhibit a richer diversity of molecular species, including complex organics and hydrocarbons, which are indicative of more advanced chemical processing and thermal conditions. The presence of these molecules in disks is often attributed to processes such as ice sublimation and gas-phase chemistry that occur at higher temperatures, facilitating the formation of more complex molecules.

What are the implications of the observed differences in molecular emission/absorption properties between sources, and how do they relate to the physical and chemical evolution of the protostellar system?

The observed differences in molecular emission and absorption properties among various low-mass protostars have profound implications for understanding the physical and chemical evolution of protostellar systems. For instance, the detection of hot water (∼1000–1200 K) in some sources indicates the presence of hot molecular gas, likely originating from embedded disks or shock interactions, suggesting that these regions are undergoing significant thermal and dynamical processes.
Moreover, the variations in molecular emission and absorption features can provide insights into the different evolutionary stages of protostellar systems. For example, sources exhibiting strong absorption features may indicate the presence of cooler, denser material, while those with prominent emission lines may suggest active outflows or disk winds. The differences in molecular ratios with respect to H2O between ice and gas phases also hint at processes such as ice sublimation and high-temperature gas-phase chemistry, which are critical for understanding the transition from protostellar to protoplanetary phases.
These findings underscore the complexity of chemical evolution in protostellar systems, where the interplay between temperature, density, and molecular composition shapes the pathways of star and planet formation.

Could the detection of unexpected molecules like SiO, CS, and NH3 at mid-IR wavelengths provide new insights into the chemical processes occurring in the inner regions of low-mass protostars?

The detection of unexpected molecules such as SiO, CS, and NH3 at mid-IR wavelengths indeed offers new insights into the chemical processes occurring in the inner regions of low-mass protostars. These molecules are typically associated with high-temperature environments and shock chemistry, suggesting that their presence indicates active physical processes such as outflows, shocks, or interactions between the protostar and its surrounding material.
For instance, SiO is often linked to shock-induced chemistry, where it can be produced through the sputtering of silicate grains in the presence of high-energy events like outflows. The detection of SiO in low-mass protostars implies that these systems may experience significant shock activity, which can enhance the chemical complexity and lead to the formation of various molecular species.
Similarly, the presence of CS and NH3 can provide clues about the thermal and chemical conditions in the inner regions of protostars. CS is a key molecule in the study of sulfur chemistry, while NH3 is crucial for understanding nitrogen chemistry in these environments. Their detection at mid-IR wavelengths suggests that the inner regions of low-mass protostars are not only chemically rich but also dynamically active, with ongoing processes that influence the overall evolution of the protostellar system.
In summary, the detection of these unexpected molecules enhances our understanding of the intricate chemical networks and physical conditions present in the early stages of star formation, highlighting the importance of mid-IR observations in probing the molecular landscape of low-mass protostars.