The infrared extinction law in various interstellar environments

1. The infrared extinction law in various interstellar environments Shu Wang 11, 30, 2012 Beijing Normal University Email: shuwang@mail.bnu.edu.cn

2. Outline • Background • Data • Selected typical environments • Method • Tracers Selection • Result and Discuss • relative extinction Aλ/Aks • varying with environments

3. Background • The infrared extinction law: wavelength, interstellar environments • The wavelength-dependent extinction • The extinction in different interstellar environments NIR: MIR: Our work: whether the IR extinction law relates to the interstellar environment or not and how it relates

4. Data • Near-infrared:Two Micron All Sky Survey (2MASS) • J (1.24 μm), H (1.66μm), 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

5. Selected 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

6. Coalsack • 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)

7. the visual extinction map visual extinction is Av, the dust column density is N, the dust volume density is n, the distance of the sight line is L AV large region: orange AVtransition region: green Fig. 1.— A sample extinction map of the Coalsack by Dobashi et al. (2005). The resolution of the map is 6′.

8. The Spitzer/IRAC image 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 μm emission region is marked with violet box.

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

10. Infrared dark cloud • 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

11. Infrared dark cloud Fig. 4.— The MSX 8.3 μm 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.

13. 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 /Akscan derive

14. Tracers 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

15. The results and discussion 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.

16. 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 3σcriterion.

17. Abnormal

18. Fig. 7.— Comparing sub-regions value with each other, the diffuse (1) region has the highest relative extinction and the 8 μm 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.

19. 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.

20. Summary • 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

21. Thank you