The infrared extinction law in The infrared extinction law in various interstellar various interstellar

environmentsenvironments

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Shu Wang 11, 30, 2012

Beijing Normal University Email: shuwang@mail.bnu.edu.cn

OutlineOutline

Background Data Selected typical environments Method Tracers Selection Result and Discuss• relative extinction Aλ/Aks

• varying with environments

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The infrared extinction law: wavelength, interstellar environments• The wavelength-dependent extinction• The extinction in different interstellar environments

Our work: whether the IR extinction law relates to the interstellar environment or not and how it relates

NIR: MIR:

BackgroundBackground

- 1.6 1.8

vary in different sight lines

A ，

DataData

Near-infrared: Two Micron All Sky Survey (2MASS)• J (1.24 μm), H (1.66 μm), and Ks (2.16 μm)

Mid-infrared: Spitzer Legacy Program, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE)• four IRAC bands

[3.6], [4.5], [5.8] and [8.0] μm

cross-identified these two catalogs

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Selected typical environmentsSelected typical environments

The Coalsack nebula three kinds of criteria are used to classify the

Coalsack into sub-regions• the visual extinction map (Dobashi et al. 2005)

• the Spitzer’s image dust emission region

• the CO gas emission image (Dame et al. 2001).

Infrared dark cloud Diffuse region• Lack of dust

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CoalsackCoalsack

located: l=303°, b= 0° distance: 150 pc∼ linear extent: 15 pc (Nyman 2008) mass of the cloud: 3500 M∼ ⊙

average AV 5 mag (Gegorio-Hetem et al. 1988) ∼

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the visual extinction mapthe visual extinction map

Fig. 1.— A sample extinction map of the Coalsack by Dobashi et al. (2005). The resolution of the map is 6′.

AV large region: orangeAV transition region: green

visual extinction is Av, the dust column density is N, the dust

volume density is n, the distance of the sight line is L

The The SpitzerSpitzer/IRAC image/IRAC image

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Fig. 2.— The Spitzer 8.0 μm image of a part of the Coalsack around l = 305.5°. The resolution of the image mosaics is 1.2 arcsec per pixel. The black filaments are due to dust emission, specially the PAH emission. The selected 8 μ m emission region is marked with violet box.

The CO gas emission imageThe CO gas emission image

Fig. 3.— Map of the velocity integrated CO (1-0) emission of Coalsack (Nyman et al. 1989).

infrared dark clouds (IRDCs)• contain extremely dense region • the Av can reach above 20 mag, even 100 mag• opaque even at 8 µm • considered as the early stages of star formation

region candidates Simon et al. (2006) using the MSX 8.3 µm images have

identified over 10000 IRDC candidates in the first and fourth quadrants of the Galactic plane.

Selected an IRDC G028.23-00.19

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Infrared dark cloudInfrared dark cloud

Infrared dark cloudInfrared dark cloud

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Fig. 4.— The MSX 8.3 μ m image of a part around l = 28°. The IRDCs and cores identified by Simon et al. (2006) are marked with black ellipses and white ellipses respectively. The cloud G028.23-00.19 marked with blue ellipse is shown on the image.

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Measurement of extinction Measurement of extinction method : “color-excess” Calculates the ratio of two color excesses: kx

The relative extinction

Drawing the J-KS versus KS-[λ] color-color diagrams

• Linear fitting to get the slopes kx

• Adopt AJ/AKs =2.52 (Rieke & Lebofsky 1985)

• Aλ /Aks can derive

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Tracers SelectionTracers Selection extinction tracers: Red Giants • scatter of the intrinsic color indices is small• bright in the IR

2MASS JHKs and Spitzer-IRAC bands ： S/N≥ 10 color restrictions • J-Ks ≥ 1.2 and H-Ks ≥ 0.3 (Gao et al. 2009)

excludes foreground dwarf stars• [3.6]−[4.5] ≤ 0.6 mag and [5.8]−[8.0] ≤ 0.2 mag

excludes pre-main-sequence stars and asymptotic giant branch (AGB) stars and young stellar objects (YSOs) (Flaherty et al. 2007)

A deviation of < 3σ

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Fig. 5.— J-Ks vs. J diagram for the 8 μm emission region in Table 1. The black dots are all the sources for the field of the GLIMPSE I which contains the 8 μm emission region and red crosses are the selected red giants for the 8 μm emission region.

The results and discussionThe results and discussion

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Fig. 6.— The 2MASS and IRAC color-color diagrams for the sources in Figure 5. The green lines are the best fits to the data. The blue points are dropped by a 3σ criterion.

Abnormal

Fig. 7.— Comparing sub-regions value with each other, the diffuse (1) region has the highest relative extinction and the 8 μ m emission region has the lowest value. It is well consistent with the experience: the relative extinction in diffuse region is higher and in dense region is lower in four IRAC bands. It may be explained by the theory of dust growth in dense region.

Fig. 8.— Comparing the mean extinction with previous results and the extinction curves calculated from the interstellar grain model for Rv = 3.1 (solid line) and for Rv = 5.5 (dot-dashed line) by Weingartner & Draine (2001). It is found all the regions have flat mid-IR extinction curve which are consistent with the modeled extinction curve of Rv =5.5.

SummarySummary

The relative extinction in the mid-infrared Aλ/AKs varies with various interstellar environments • In diffuse region is higher than that in dense region• Dust growth theory

The mean extinction laws are consistent with the modeled extinction curve of RV =5.5

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