4.3 Protostellar objects and their chemical composition

Protostellar objects mark the start of stellar evolution. They're born when molecular clouds collapse, forming young stars surrounded by gas and dust envelopes. These objects evolve through different stages, classified by their spectral energy distribution and mass distribution.

The chemical makeup of protostellar envelopes and disks is complex. It varies with distance from the central star, influenced by temperature and density gradients. Gas-phase and dust grain chemistry play crucial roles in shaping the molecular composition of these environments.

Protostellar Objects and their Characteristics

Classification and Evolution of Protostars

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• Protostellar objects represent the earliest stage of stellar evolution where a molecular cloud core collapses under its own gravity to form a young stellar object (YSO)
• The protostellar phase is characterized by a central protostar surrounded by an infalling envelope of gas and dust, with a circumstellar disk forming around the protostar
• Protostars are classified into different stages based on their spectral energy distribution (SED) and the relative masses of the envelope, disk, and protostar
  • Class 0 protostars are deeply embedded in their envelopes, with most of the mass still in the envelope and a very low protostellar mass (e.g., VLA 1623, L1527)
  • Class I protostars have accreted a significant portion of their final mass, with a more massive protostar and a less massive envelope (e.g., L1551 IRS 5, Elias 29)
  • Class II and III objects represent the later stages of protostellar evolution, with the envelope dissipated and the circumstellar disk becoming more prominent (e.g., T Tauri stars, Herbig Ae/Be stars)

Observational Characteristics of Protostellar Objects

• Protostellar objects emit a significant portion of their energy in the infrared due to the presence of the cold, dusty envelope and disk surrounding the central protostar
• The infrared excess in the SED of protostellar objects is a key observational signature that distinguishes them from more evolved pre-main sequence stars
• Protostellar objects are often associated with molecular outflows and jets, which are driven by the accretion process and can be observed in molecular lines (CO) and shocked H2 emission
• Protostellar objects are typically located in dense cores within molecular clouds, and their positions can be determined through observations of dust continuum emission and molecular line emission (NH3, N2H+)

Chemical Composition of Protostellar Envelopes and Disks

Gas-Phase Chemistry in Protostellar Envelopes and Disks

• The gas in protostellar envelopes and disks is mainly composed of molecular hydrogen (H2), with smaller amounts of other molecules such as CO, H2O, NH3, CH4, and CO2
• The chemical composition of the gas varies with distance from the central protostar, with temperature and density gradients leading to the formation of distinct chemical regions
  • The inner regions of protostellar envelopes and disks are characterized by higher temperatures and densities, leading to the evaporation of ices and the formation of more complex organic molecules (CH3OH, HCOOH)
  • The outer regions of protostellar envelopes and disks are colder and less dense, allowing for the freeze-out of molecules onto dust grains and the formation of icy mantles
• The gas-phase chemistry in protostellar envelopes and disks is driven by ion-molecule reactions, neutral-neutral reactions, and photochemistry, with the relative importance of these processes depending on the physical conditions and the availability of ionizing radiation

Dust Grain Chemistry in Protostellar Envelopes and Disks

• Dust grains in protostellar envelopes and disks are composed of silicates, carbonaceous material, and ices, with the ice mantles containing a variety of simple molecules such as H2O, CO, CO2, CH3OH, and NH3
• The composition of the ice mantles on dust grains depends on the temperature and density of the surrounding gas, with different molecules freezing out at different temperatures
  • H2O ice is the most abundant ice component and freezes out at temperatures below ~100 K
  • CO ice freezes out at temperatures below ~20 K, while more complex molecules like CH3OH and HCOOH require even lower temperatures (<10 K) to freeze out efficiently
• The chemical composition of the ice mantles can be altered by energetic processing, such as cosmic ray bombardment, UV photolysis, and thermal processing, leading to the formation of more complex organic molecules
• The thermal desorption of ices from dust grains plays a crucial role in the chemical evolution of protostellar envelopes and disks, with the evaporation of icy mantles enriching the gas-phase abundances of various molecules

Accretion and Outflows in Protostar Evolution

Impact of Accretion on Protostellar Chemistry

• Accretion, the process by which material from the protostellar envelope and disk falls onto the central protostar, releases gravitational energy and heats up the surrounding gas
• Accretion shocks at the protostellar surface can drive high-temperature chemistry, leading to the formation of complex organic molecules and the destruction of simpler species
  • The high temperatures in accretion shocks can overcome energy barriers for chemical reactions, enabling the formation of molecules that are not typically produced in cold environments (e.g., HCN, C2H2)
  • The destruction of simpler molecules in accretion shocks can alter the overall chemical composition of the inner envelope and disk
• Episodic accretion events, such as FU Orionis outbursts, can dramatically alter the chemical composition of the inner envelope and disk by heating up the gas and evaporating icy mantles
  • During an FU Orionis outburst, the luminosity of the protostar can increase by several orders of magnitude, leading to a rapid increase in the temperature of the surrounding material
  • The evaporation of icy mantles during an outburst can release a large amount of molecules into the gas phase, significantly altering the chemical composition of the inner envelope and disk

Role of Outflows in Shaping Protostellar Chemistry

• Outflows are high-velocity jets and winds launched from the protostellar system, which can impact the chemistry of the surrounding material
• Outflows can create shocks and cavities in the protostellar envelope, altering the density structure and exposing material to higher temperatures and radiation fields
  • Shocks driven by outflows can heat the gas to temperatures of several thousand Kelvin, enabling the formation of high-temperature molecules like SiO and SO
  • Cavities created by outflows allow UV radiation from the protostar to penetrate deeper into the envelope, driving photochemical reactions and altering the molecular abundances
• Shocks driven by outflows can lead to the sputtering of dust grains and the release of molecules from icy mantles, enhancing the gas-phase abundances of certain species
  • The sputtering of dust grains in outflow shocks can release refractory elements like Si and Fe into the gas phase, enabling the formation of molecules like SiO and FeO
  • The release of molecules from icy mantles in outflow shocks can enhance the gas-phase abundances of species like H2O, CH3OH, and HCOOH
• Outflows can transport chemically processed material from the inner regions of the protostellar system to the outer envelope and surrounding molecular cloud
  • The entrainment of material by outflows can carry molecules formed in high-temperature regions near the protostar to cooler, more quiescent regions of the envelope
  • The mixing of chemically processed material with the surrounding molecular cloud can lead to the enrichment of the cloud with complex organic molecules and other species not typically found in cold, dense environments

Deuterium Fractionation in Protostellar Objects

Mechanism and Efficiency of Deuterium Fractionation

• Deuterium fractionation is the enrichment of deuterated molecules relative to their non-deuterated counterparts, which occurs through ion-molecule reactions that favor the incorporation of deuterium into molecules at low temperatures (typically below 30 K)
• The main reaction pathway for deuterium fractionation involves the transfer of a deuteron (D+) from H2D+ to other molecules, such as CO and N2, leading to the formation of deuterated species like DCO+ and N2D+
  • The formation of H2D+ is favored at low temperatures due to the lower zero-point energy of the deuterated isotopologue compared to the non-deuterated form
  • The degree of deuterium fractionation depends on the ortho-to-para ratio of H2, with a lower ortho-to-para ratio leading to enhanced deuteration
• Deuterium fractionation is most efficient in the cold, dense regions of protostellar envelopes and disks, where CO and other heavy molecules are depleted from the gas phase due to freeze-out onto dust grains
  • The depletion of CO enhances the abundance of H2D+ and other deuterated ions, as CO is one of the main destroyers of these species
  • The low temperatures and high densities in these regions promote the formation of deuterated molecules through ion-molecule reactions

Using Deuterium Fractionation as a Probe of Protostellar Chemistry and Evolution

• The abundance of deuterated molecules can be used as a tracer of the temperature and density structure of protostellar objects, as well as the degree of CO depletion
  • High abundances of deuterated molecules, such as D2CO and CH2DOH, are indicative of cold, dense regions where CO is heavily depleted (e.g., L1544, IRAS 16293-2422)
  • The deuterium fractionation of different molecules can provide information about the relative ages of protostellar cores, with more evolved objects typically showing lower levels of deuteration
• Studying deuterium fractionation in protostellar objects is crucial for understanding the initial conditions and chemical history of star-forming regions
  • The degree of deuterium fractionation in protostellar envelopes and disks can constrain the duration of the starless core phase and the timescales for protostellar collapse
  • The spatial distribution of deuterated molecules can provide insights into the physical structure and evolutionary stage of protostellar objects
• Deuterium fractionation in protostellar objects is also relevant for understanding the origin of deuterium enrichment in solar system bodies, such as comets and meteorites
  • Comets and meteorites often exhibit high levels of deuterium enrichment in organic molecules, which is thought to be inherited from the cold, dense regions of the protostellar envelope where these objects formed
  • Studying deuterium fractionation in protostellar objects can help establish the link between the chemical composition of the early solar system and the conditions in the protostellar environment