Mid-infrared extinction and fresh silicate dust towards the Galactic Center

TL;DR

This study models the mid-infrared extinction towards the Galactic Center, proposing a dust composition of aromatic carbon and layered silicate particles with magnetite, to explain the flat extinction feature observed.

Contribution

It introduces a new dust model using laboratory-measured optical constants and layered particles, improving the understanding of Galactic Center extinction.

Findings

A mixture of aromatic carbon and layered silicate particles fits the extinction data.

Metallic iron is likely embedded within dust particles, not layered on silicates.

The dust is likely recently formed in late-type star atmospheres within the Galactic Center.

Abstract

We interpret the interstellar extinction observed towards the Galactic Center (GC) in the wavelength range $\lambda = 1 - 20\,\mkm$. Its main feature is the flat extinction at $3 - 8\,\mkm$ whose explanation is still a problem for the cosmic dust models. We search for structure and chemical composition of dust grains that could explain the observed extinction. In contrast to earlier works we use laboratory measured optical constants and consider particles of different structure. We show that a mixture of compact grains of aromatic carbon and of some silicate is better suited for reproducing the flat extinction in comparison with essentially porous grains or aliphatic carbon particles. Metallic iron should be located inside the particle, i.e. cannot form layers on silicate grains as the extinction curves become then very peculiar. We find a model including aromatic carbonaceous particles and three-layered particles with an olivine-type silicate core, a thin very porous layer and a thin envelope of magnetite that provides a good (but still not perfect) fit to the observational data. We suggest that such silicate dust should be fresh, i.e. recently formed in the atmospheres of late-type stars in the central region of the Galaxy. We assume that this region has a radius of about 1 kpc and produces about a half of the observed extinction. The remaining part of extinction is caused by a "foreground" material being practically transparent at $\lambda = 4 - 8\,\mkm$.

Tables

Table 1. Table 2: Best-fit dust models

$N$	Model components	${\chi }^2$/d.o.f.	$A^{\ast }\left(7.5\mu m\right)$	$R_V$	${\lambda }_m$	$A^{\ast }\left({\lambda }_m\right)$	Figs.
	Observations	–	0.81	$\lesssim 3$:	9.6	3.45	Figs. 1–4
Homogeneous spheres
1	astrosil (${𝒦}_{\mathrm{Si}}=0.50$) / ACBE$\_$zu	36.4	0.561	3.07	9.5	3.43	Fig. 1
2	olmg50 (${𝒦}_{\mathrm{Si}}=0.25$) / cell400	116.4	0.320	3.25	9.8	3.41	Fig. 1
3	olmg50 (${𝒦}_{\mathrm{Si}}=0.55$) / cell1000	36.6	0.321	3.26	9.8	3.36	Fig. 1
4	olmg40 (${𝒦}_{\mathrm{Si}}=0.77$) / cell1000	112.9	0.261	3.29	9.8	3.46
5	olmg100 (${𝒦}_{\mathrm{Si}}=0.56$) / cell1000	36.1	0.706	2.61	9.7	3.46
6	pyrmg50 (${𝒦}_{\mathrm{Si}}=0.47$) / cell1000	47.8	0.381	2.87	9.2	3.47	Fig. 1
7	pyrmg40 (${𝒦}_{\mathrm{Si}}=0.48$) / cell1000	43.1	0.386	3.00	9.0	3.49
8	pyrmg100 (${𝒦}_{\mathrm{Si}}=0.39$) / cell1000	74.9	0.385	2.73	9.4	3.48
9	OHM-SiO (${𝒦}_{\mathrm{Si}}=0.92$) / cell1000	35.4	0.503	4.02	10.0	3.41
10	olmg50 (${𝒦}_{\mathrm{Si}}=0.44$) / H2O (${𝒦}_{H_{2}O}=0.20$) / cell1000	30.4	0.420	2.98	9.8	3.42
11	olmg50 (${𝒦}_{\mathrm{Si}}=0.50$) / Fe (${𝒦}_{\mathrm{Fe}}=0.20$) / cell1000	87.8	0.208	3.94	9.8	1.84
EMT-Mie calculations
12	80%olmg50+20% vac (${𝒦}_{\mathrm{Si}}=0.53$) / cell1000	32.9	0.338	2.64	10.0	2.86
13	olmg50 (${𝒦}_{\mathrm{Si}}=0.46$) / 80%cell1000+20% vac	26.4	0.383	3.34	9.8	2.78
14	a-SilFe (${𝒦}_{\mathrm{Si}}=0.40$) / cell1000	37.2	0.312	4.60	9.9	1.40
15	a-SilFe (${𝒦}_{\mathrm{Si}}=0.34$) / optEC(s)	487.0	0.080	3.92	10.0	3.41
16	amFo-10Fe30FeS (${𝒦}_{\mathrm{Si}}=0.47$) / cell1000	40.6	0.287	4.29	9.9	1.64
17	amEn-10Fe30FeS (${𝒦}_{\mathrm{Si}}=0.39$) / cell1000	36.5	0.290	5.06	9.5	1.77
Core-mantle spheres
18	20% vac–80% olmg50 (${𝒦}_{\mathrm{Si}}=0.53$) / cell1000	32.6	0.335	2.66	10.0	2.82
19	olmg50 (${𝒦}_{\mathrm{Si}}=0.44$) / 20% vac–80%cell1000	25.9	0.380	3.68	9.8	2.53
20	93% a-SilFe–7% cel1000 ($𝒦=0.28$) / 73% optEC(s)–27% cell1000	313.9	0.240	3.48	10.0	3.44	Fig. 4
21	93% olmg50–7% cel1000 ($𝒦=0.19$) / 73% cel400–27% cell1000	90.1	0.391	2.99	9.8	3.35
Three-layered spheres
22	10% Fe–10% vac∗–80% olmg50 ($𝒦=0.77$) / cell1000	164.3	0.231	3.06	9.8	3.41	Fig. 2
23	10% vac–10% Fe–80% olmg50 ($𝒦=0.18$) / cell1000	68.5	0.280	5.29	9.6	0.68	Fig. 2
24	98.99% olmg50–1% vac∗–0.01% Fe ($𝒦=0.09$) / cell1000	89.5	1.213	4.83	8.2	1.49	Fig. 2
25	90% olmg50–5% vac∗–5% Fe3O4 ($𝒦=0.91$) / cell1000	6.7	0.696	3.22	9.8	3.45	Fig. 3, 4
26	90% olmg50–5% vac∗–5% Fe2O3 ($𝒦=0.47$) / cell1000	31.7	0.332	3.56	9.8	2.39	Fig. 3
27	90% olmg50–5% vac∗–5% FeO ($𝒦=0.48$) / cell1000	36.2	0.319	3.51	9.8	2.21	Fig. 3
28	90% olmg50–5% vac∗–5% FeS ($𝒦=0.16$) / cell1000	94.3	0.319	4.42	9.8	0.61	Fig. 3
29	90% pyrmg50–5% vac∗–5% Fe3O4 ($𝒦=0.72$) / cell1000	75.7	0.781	2.91	9.1	3.22
Two-cloud model
30	model 20 + model 25 (see Sect. 3.4)	84.9	0.468	3.32	9.9	3.41	Fig. 4

Table 2. Table 3: Sources of optical constants

Notation	Material	Reference
astrosil	astronomical silicate	Draine (2003)
olmg50	amorphous olivine (MgFeSiO4)	Dorschner et al. (1995)
olmg40	amorphous olivine (Mg0.8Fe1.2SiO4)	Dorschner et al. (1995)
olmg100	amorphous olivine (Mg2SiO4, forsterite)	Jäger et al. (2003a)
pyrmg50	amorphous pyroxene (Mg0.5Fe0.5SiO3)	Dorschner et al. (1995)
pyrmg40	amorphous pyroxene (Mg0.4Fe0.6SiO3)	Dorschner et al. (1995)
pyrmg100	amorphous pyroxene (MgSiO3, enstatite)	Dorschner et al. (1995)
OHM-SiO	O-rich interstellar silicate	Ossenkopf et al. (1992)
a-SilFe	amorphous olivine (MgFeSiO4+10%Fe)	Jones et al. (2013)
amFo-10Fe30FeS	amorphous forsterite (Mg2SiO4+10%Fe+30%FeS)	Köhler et al. (2014)
amEn-10Fe30FeS	amorphous enstatite (MgSiO3+10%Fe+30%FeS)	Köhler et al. (2014)
ACBE$\_$zu	amorphous carbon (type BE)	Zubko et al. (1996)
cell400	pyrolizing cellulose ($T=400°$C, aliphatic, a-C(:H))	Jäger et al. (1998)
cell1000	pyrolizing cellulose ($T=1000°$C, aromatic, a-C)	Jäger et al. (1998)
optEC(s)	amorphous carbon (a-C(:H), band gap $E_g=2.5$ eV)	Jones (2012)
Fe	iron	Jones et al. (2013)
FeO	wüstite	Henning et al. (1995)
Fe2O3	hematite	Jena laboratory
Fe3O4	magnetite	Jena laboratory
FeS	troilite	Pollack et al. (1994)
H2O	water ice	Warren & Brandt (2008)

Equations

$$A^{\ast }(\lambda )=\frac{A(\lambda )}{A_{\mathrm{Ks}}}2\text{fm}49.$$

$$A_{\mathrm{total}}^{\ast }(\lambda )=f{A}_{\mathrm{foregr}}^{\ast }(\lambda )+(1-f)A_{\mathrm{backgr}}^{\ast }(\lambda ),$$

Full text

dust, extinction — Galaxy: center, clouds

\AuthorCallLimit

=5

1 Introduction

The center parts of the Milky Way are a unique place to study different processes in the vicinity of a supermassive black hole as well as dynamics and star formation under extreme conditions (Genzel et al., 2010; Mapelli & Gualandris, 2016). The Galactic Center (GC)111Hereafter, by the Galactic Center we mean a central region of about 1 kpc radius. invisible at the optical wavelengths can be observed in the infrared (IR) where the extinction amounts to $A_{\rm Ks}=2\fm 49$ (Fritz et al., 2011). A distinguishing feature of the GC extinction is its flat wavelength dependence at $3\,\mu{\rm m}<\lambda<8\,\mu{\rm m}$. The flat or gray extinction in the GC was firstly measured by Lutz et al. (1996) with ISO, using hydrogen recombination lines, and confirmed by Lutz (1999), Nishiyama et al. (2009), and Fritz et al. (2011). Numerous recent observations appear to suggest the universality of flat extinction in the mid-IR for both diffuse and dense environments (see Wang et al., 2013,  for a summary).

Fritz et al. (2011) have compared different dust models capable of explaining the mid-IR extinction in the GC. The models were from Weingartner & Draine (2001) (mixture of carbonaceous and silicate spheres), Zubko et al. (2004) (mixture of carbonaceous and silicate particles and additionally composite grains consisting of silicates, organic refractory material, water ice, and voids222The optical properties of such particles were calculated using the Mie theory for homogeneous spheres and refractive indexes averaged according to the Effective Medium Theory (EMT).), Dwek (2004) (mixture of bare particles of Zubko et al. (2004) and additionally metallic needles), and Voshchinnikov et al. (2006) (multi-layered spheres consisting of silicate, carbon, and vacuum). Wang et al. (2014) have developed the idea of Dwek (2004) and considered additionally micrometer-sized particles from amorphous carbon, graphite, silicate or iron. Such a model with amorphous carbon explained the flat extinction at $3-8\,\mu{\rm m}$, but required the solid-phase C abundance C/H$=$352 ppm that exceeded the solar abundance of carbon (269 ppm, Asplund et al., 2009). An important feature of the modelings mentioned above is a priori selection of the optical constants of grain materials. Moreover, all the authors used the optical constants of the “astronomical silicate” (astrosil) obtained by empirical fits to some observations by Draine & Lee (1984). The imaginary part of the complex refractive index of astrosil $k$ slightly grows with $\lambda$ in the region $3-8\,\mu{\rm m}$, which does not coincide with the behaviour of $k$ for any silicate material (see Fig. A.1 in Jones et al., 2013).

It should be emphasized that there are no cosmic dust models that can explain the flat (excess) mid-IR extinction observed in the GC and other galactic objects. The COMP-AC-S model of Zubko et al. (2004) gives a good fit, but produces strong 3 $\mu{\rm m}$ band, which disagrees with the trend found in the Coalsack nebula Globule 7 by Wang et al. (2013). The most recent model of Wang et al. (2015) includes $4\,\mu{\rm m}$ clean water ice particles and does explain both mid-IR extinction and the abundance of oxygen in dust, but the ice particles hardly can be so large and clean in the interstellar medium (ISM).

In this paper, we analyze a large set of dust models, concentrating on variations of grain structure and a proper presentation of grains’ chemical composition, to find a model that fits the near- and mid-IR extinctions and the 10 $\mu{\rm m}$ feature observed to the GC. The next section contains a description of the observational data and the models. The results and their discussion are presented in Sect. 3. Concluding remarks are given in Sect. 4.

2 Observational data and dust model

The GC extinction has been observationally obtained by Fritz et al. (2011) (the region 1.3–19 $\mu{\rm m}$), Nishiyama et al. (2009) (1.2–8 $\mu{\rm m}$), and Chiar & Tielens (2006) (1.2–25 $\mu{\rm m}$). The latter paper contains the probable extinction profile of the 9.7 $\mu{\rm m}$ silicate feature for the GC. All data have been normalized by us in order to have $A_{\rm Ks}=2\fm 49$ at $\lambda_{\rm Ks}=2.17\,\mu{\rm m}$,

[TABLE]

They are plotted in all Figures below.

It should be note that the GC extinction was estimated from observations in different ways. As a result, the data of Fritz et al. (2011) were mainly derived for the central 14${}^{\prime\prime}\times$20*′′* region, the data of Nishiyama et al. (2009) are averaged over the region $|l|<3^{\circ},|b|<1^{\circ}$, and the data of Chiar & Tielens (2006) are the extinction towards the Wolf-Rayet star WR 98a ($l\approx 358^{\circ},b\approx 0^{\circ}$) extended to the line of sight to GCS3. However, the extinction law for $\lambda<14\,\mu{\rm m}$ is practically the same. Hence, the data can be combined, and the question on where is located the dust that produces the extinction is not as important as it could be.

We base our analysis on the model of Hirashita & Voshchinnikov (2014) who chose the initial size distributions of silicate and carbonaceous dust that fited the mean Milky Way extinction curve with $R_{V}=3.1$ (Weingartner & Draine, 2001) and considered dust grain size evolution due to the accretion and coagulation processes.

So, our model contains two populations of grains: silicate (Si) and carbonaceous (C) ones333To reproduce the 2175Å  feature small graphite spheres were also involved (see, e.g., Das et al., 2010). with the size distributions from Hirashita & Voshchinnikov (2014). As extinction only weekly depends on the particle shape (Voshchinnikov & Das, 2008), we assume that dust grains are spherical.

Thus, the model has the following parameters:

1. the chemical composition of silicate and carbonaceous particles;
2. the structure of particles;
3. the relative number of silicate grains ${\cal K}_{\rm Si}=N_{\rm Si}/N_{\rm dust}$, where $N_{\rm Si}$ and $N_{\rm dust}$ are the column densities of silicate grains and all dust particles, respectively;
4. the time of evolution. Sometimes, we also included an additional population of dust.

When considering the chemical composition, we mainly oriented on the optical constants obtained in Jena laboratory (http://www.astro-uni-jena.de/Laboratory/). Information about these and many other data is collected in the Heidelberg–Jena–Petersburg Database of Optical Constants (HJPDOC) described by Henning et al. (1999) and Jäger et al. (2003b). The materials used for our modelling are outlined in Table 4 in the Appendix.

For homogeneous spheres, the extinction efficiency factors were calculated with the Mie theory. For composite particles, the factors were computed by using the Mie theory and the Bruggeman mixing rule of the EMT or the theory for multi-layered spheres (see Voshchinnikov & Mathis, 1999).

3 Results and discussion