[Paper Review] ISO far infrared observations of the high latitude cloud L1642. II. Correlated variations of far-infrared emissivity and temperature of "classical large" dust particles

K. Lehtinen, M. Juvela+3 more|ArXiv.org|Jan 26, 2007

Astrophysics and Star Formation Studies30 references19 citations

TL;DR

This study uses ISO far-infrared and optical extinction data to analyze dust emissivity and temperature variations in the high-latitude cloud L1642. It finds that apparent emissivity increases by a factor of ~2 as dust temperature drops from 19 K to 14 K, and radiative transfer modeling shows that a 2–3× increase in far-IR absorption cross-section is required to explain both the temperature decrease and emissivity rise in dense regions, indicating grain coagulation or ice mantle growth beyond simple radiation shielding.

ABSTRACT

Our aim is to compare the infrared properties of big, ``classical'' dust grains with visual extinction in the cloud L1642. In particular, we study the differences of grain emissivity between diffuse and dense regions in the cloud. The far-infrared properties of dust are based on large-scale 100um and 200um maps. Extinction through the cloud has been derived by using the star count method at B- and I-bands, and color excess method at J, H and Ks bands. Radiative transfer calculations have been used to study the effects of increasing absorption cross-section on the far-infrared emission and dust temperature. Dust emissivity, measured by the ratio of far-infrared optical depth to visual extinction, tau(far-IR)/A(V), increases with decreasing dust temperature in L1642. There is about two-fold increase of emissivity over the dust temperature range of 19K-14K. Radiative transfer calculations show that in order to explain the observed decrease of dust temperature towards the centre of L1642 an increase of absorption cross-section of dust at far-IR is necessary.This temperature decrease cannot be explained solely by the attenuation of interstellar radiation field. Increased absorption cross-section manifests itself also as an increased emissivity. We find that, due to temperature effects, the apparent value of optical depth tau(far-IR), derived from 100um and 200um intensities, is always lower than the true optical depth.

Motivation & Objective

• To investigate how far-infrared emissivity and dust temperature vary across different regions of the high-latitude dark cloud L1642.
• To determine whether the observed decrease in dust temperature toward the cloud's center can be explained by interstellar radiation field attenuation alone.
• To assess whether changes in dust grain properties—such as coagulation or ice mantle growth—are necessary to explain the observed correlation between temperature and emissivity.
• To distinguish between apparent and true values of optical depth, emissivity, and temperature derived from 100 μm and 200 μm intensity maps.
• To quantify the required increase in far-IR absorption cross-section to reconcile observed temperature and emissivity trends with radiative transfer models.

Proposed method

• Large-scale 100 μm and 200 μm far-infrared maps from ISO and IRAS were used to derive apparent dust temperature and optical depth.
• Visual extinction was measured using star count methods in B and I bands and color excess techniques in J, H, and Ks bands from the 2MASS survey.
• The apparent emissivity was calculated as the ratio of far-IR optical depth to visual extinction (τ(far-IR)/AV), with corrections applied for background emission.
• Radiative transfer models were employed to simulate the effects of increasing dust absorption cross-section at far-IR wavelengths on observed temperature and emissivity.
• Apparent parameters were compared to true values by simulating intensity maps from model dust grains and deriving apparent optical depth and emissivity from those simulations.
• The study used a 2.5× scaling factor derived from model simulations to convert apparent emissivity to true emissivity, enabling comparison with theoretical dust models.

Experimental results

Research questions

• RQ1 Does the observed decrease in dust temperature toward the center of L1642's dense region B exceed what can be explained by attenuation of the interstellar radiation field alone?
• RQ2 How does the apparent far-infrared emissivity (τ(far-IR)/AV) vary with dust temperature across different regions of L1642?
• RQ3 What increase in far-IR absorption cross-section is required to simultaneously explain the observed temperature drop and emissivity increase in region B?
• RQ4 Why are apparent optical depth and emissivity systematically lower than their true values in the presence of temperature bias?
• RQ5 Can grain coagulation or ice mantle accretion explain the observed emissivity enhancement, and which mechanism is more consistent with the observed 2–3× increase in absorption cross-section?

Key findings

• Apparent emissivity (τ(far-IR)/AV) increases by a factor of approximately 2 as dust temperature decreases from 19 K to 14 K across regions A, B, and C in L1642.
• The apparent optical depth and emissivity are systematically lower than their true values due to a temperature bias in the derivation from 100 μm and 200 μm intensity maps.
• A 2–3× increase in far-IR absorption cross-section is required to explain both the observed 10 K temperature drop in region B and the correlated emissivity rise.
• The true emissivity of region B (σH,true(200 μm) ≈ 3.0×10⁻²⁵ cm²) exceeds that of diffuse ISM models by a factor greater than 2.5, indicating significant grain evolution.
• The observed emissivity increase cannot be explained by ice mantle accretion alone, which increases emissivity by at most a factor of 2, suggesting grain coagulation as a more plausible mechanism.
• Radiative transfer modeling confirms that the combination of increased absorption cross-section and reduced dust temperature is necessary to reproduce the observed far-IR and extinction data in the densest parts of L1642.