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A study of iron and dust in the supernova remnant IC 443
Planetary and Space Science 116 (2015) 92–96
Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
A study of iron and dust in the supernova remnant IC 443 Takuma Kokusho a,n, Takahiro Nagayama b, Hidehiro Kaneda a, Daisuke Ishihara a, Ho-Gyu Lee c, Takashi Onaka d a
Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan Department of Astrophysics, Kagoshima University, Kagoshima 890-0065, Japan c Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea d Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2014 Received in revised form 19 February 2015 Accepted 16 April 2015 Available online 24 April 2015
We performed near-infrared line mapping of the supernova remnant IC 443 with the IRSF 1.4-m telescope, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. We detected the [Fe II] and Paβ emissions from a very wide area of the observed region (30′ × 35′). These line intensity maps reveal a global correlation between [Fe II] and Paβ. Kokusho et al. (2013) found that the [Fe II] emission is notably strong while dust emission is faint in inner regions of IC 443. These results indicate that the abundant Fe þ is likely to be of interstellar origins through shock destruction of dust, rather than of ejecta origins. From the X-ray intensity map of the 6.7 keV Fe-K line obtained with Suzaku, we find that highly ionized He-like Fe ions exist in the inner regions of IC 443, toward which Fe þ is detected. The presence of the He-like Fe ions and the faint dust emission indicates that Fe is likely to be interacting with X-ray plasma for a time long enough to be highly ionized. We discuss the implications of the detection of the lowly ionized Fe ions in such regions for the processing and composition of dust in the interstellar environment around IC 443. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Supernova remnant Interstellar dust Iron IRSF AKARI Suzaku
1. Introduction In supernova remnants (SNRs), the interstellar medium (ISM), composed of gas and dust, is heated and processed through shocks generated by supernovae, while supernovae also supply metal-rich ejecta synthesized in progenitors (e.g., Hughes et al., 2000), and dust into the interstellar space (e.g., Matsuura et al., 2011). In general, it is difficult to identify the elements depleted onto dust grains. However, in SNR shock regions, such elements incorporated in dust are released into the interstellar space through sputtering destruction of dust by X-ray plasma. Gas-phase elements thus liberated from dust can be excited in shock regions and emit spectral lines. In this way, SNRs give us an opportunity to investigate processing mechanisms and chemical compositions of dust. Fe, one of the abundant metals in the interstellar space, is thought to be heavily depleted onto dust grains (e.g., Draine, 1995). Since there is little direct evidence for the presence of Fe in dust, it is not well understood how Fe is contained in dust. For instance, it is suggested that silicate grains are generally poor in Fe (e.g., Henning, 2010), while McDonald et al. (2010) proposed that metallic Fe is formed in stellar winds. Pure Fe grains, if any, are n
Corresponding author. E-mail address: [email protected] (T. Kokusho).
http://dx.doi.org/10.1016/j.pss.2015.04.009 0032-0633/& 2015 Elsevier Ltd. All rights reserved.
difficult to identify, because they exhibit a featureless spectrum (e.g., Rho et al., 2008). In order to address such an Fe problem, SNRs, where shock-heated plasmas are sputtering away the elements contained in dust, offer valuable probes to examine forms and amounts of Fe in dust. IC 443 is one of the Galactic SNRs where shocks are strongly interacting with the surrounding ISM. The SNR is located in the direction of the Galactic anti-center at a distance of 1.5 kpc (Welsh and Sallmen, 2003), and its age is estimated to be in the range of 3000–30,000 years (Petre et al., 1988; Olbert et al., 2001). A pulsar wind nebula and metal-rich X-ray plasma are detected in the southern and northern regions of IC 443, respectively (Gaensler et al., 2006; Troja et al., 2008), indicating that the SNR is of a corecollapse origin. From limb-brightened morphologies at optical, infrared, and radio wavelengths (Bykov et al., 2008; Castelletti et al., 2011) and a centrally peaked morphology in the X-ray (Troja et al., 2008), IC 443 is categorized as a mixed-morphology-type SNR. In relation to the interaction between the shocks and the surrounding ISM in IC 443, an outstanding feature is seen in the nearinfrared (IR) wavelength. With the Two Micron All Sky Survey (2MASS), Rho et al. (2001) revealed that ionic and molecular shocks are propagating in IC 443. The northeastern shell of IC 443 is bright in the J and H bands, while it is faint in the Ks band. In this region, Graham et al. (1987) detected the [Fe II] 1.644 μm and Brγ emissions, who found that the ratio of [Fe II] 1.644 μm to Brγ is
T. Kokusho et al. / Planetary and Space Science 116 (2015) 92–96
considerably larger than that expected for HII regions, indicating a significant increase in the amount of gas-phase Fe due to the dust destruction. For the northeastern shell, Rho et al. (2001) concluded that the J band and the H band are dominated by the [Fe II] 1.257 μm and Paβ, and the [Fe II] 1.644 μm emissions, respectively. In contrast to the northeast region, the southern shell is bright only in the Ks band. With the near-IR spectroscopy, strong H2 line emission is detected from parts of the southern ridge (e.g., Richter et al., 1995; Shinn et al., 2011), and therefore, H2 seems to be a dominant career in the Ks band. In this region, Noriega-Crespo et al. (2009) detected the [Fe II] 26 μm emission with the Spitzer/ Infrared Spectrograph (IRS), which showed that faint [Fe II] filaments, such as not recognized with 2MASS, exist also in the southern region. To destroy dust and release Fe from dust into the interstellar space, fast shocks ( ≥ 150 km s−1) are needed (Jones et al., 1994), while H2 is dissociated by such fast shocks. Therefore, from the above near-IR morphology of IC 443, fast shocks are dominant in the northern shell, while shocks seem to be decelerated by the dense ISM in the southern ridge, where molecular clouds are distributed (e.g., Lee et al., 2012). In this paper, in order to study Fe and dust in IC 443, we focus on the spatial distributions of [Fe II] and Paβ in the near-IR, and X-ray 6.7 keV Fe-K (He-like ion) line emission as well as mid- and far-IR dust continuum emission in IC 443. To investigate the spatial distributions of the [Fe II] and Paβ emissions, we performed line mapping of IC 443 by a ground-based telescope. Comparing these maps with those in dust continuum and 6.7 keV Fe-K line emission obtained by space telescopes, we examine dust processing and composition through shock destruction in the SNR. Table 1 Log of the Suzaku observations. Observation ID
Date
(R.A., decl.)J2000
Exposure time (k sec)
501006020 507015020 507015030 507015040
7 March 2007 27 March 2013 31 March 2013 6 April 2013
(94.2972, (94.3028, (94.3024, (94.3024,
44 59 131 76
22.4797) 22.7465) 22.7479) 22.7479)
Counts/s/keV
2. Observation and data reduction
pixel scale of the SIRIUS camera are 7′.7 × 7′.7 and 0″.45, respectively. In order to investigate the main shell structure of IC 443, we observed 44 overlapped field of views, covering a large area of
10−3
10−4 4 2 χ
Fig. 2. Pseudo-color image of IC 443 (blue: [Fe II] 1.257 μm, red: Paβ, both for a range of 1.0 × 10−5–2.0 × 10−4 ergs s−1 cm−2 sr−1), where point sources are removed and the contour map of the warm dust emission is superimposed with the levels of 0.5, 1.5, 2.5, and 3.5 × 10−3 ergs s−1 cm−2 sr−1. The color images are smoothed with a Gaussian kernel of σ = 2″.3. The original data for the [Fe II] 1.257 μm and the warm dust emission are taken from Kokusho et al. (2013). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
We carried out line mapping of IC 443 with the near-IR camera SIRIUS (Simultaneous InfraRed Imager for UnbiasedSurvey; Nagashima et al., 1999; Nagayama et al., 2003) on the IRSF (InfraRed Survey Facility) 1.4-m telescope, located at the South African Astronomical Observatory, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. A field of view and a
0.01
0 −2 −4
93
4.5
5
5.5
6
6.5
7
Energy (keV) Fig. 1. XIS spectrum detected with the Suzaku/XIS from a local area (4′.4 × 4′.4 ) at (R. A ., decl. ) = (06h17m 29s , 22°47′33″), and the result of spectral fitting for an energy range of 4.0–7.3 keV (reduced χ 2 = 0.87). The top panel shows the best-fit model composed of bremsstrahlung continuum, 6.7 keV Fe-K line, and absorbed power-law (the cosmic X-ray background) components overlaid on the spectrum with the error bar of 1s. The bottom panel shows the residuals between the data and fitted model.
30′ × 35′. The details of the observation for the [Fe II] emission are described in Kokusho et al. (2013). We observed the Paβ emission in November 2012 and February 2013, with exposure time of 60 s and the dithering number of 10. For data reduction, we applied the same standard procedure as explained in Kokusho et al. (2013). For the flux calibration of the Paβ intensity map, we used the J-band data, assuming that the magnitudes of stars are the same between the narrow- and broad-band images. We analyzed the mid- and far-IR data obtained by AKARI and Spitzer, and the X-ray data by Suzaku. The details of the AKARI observation and data reduction are described in Kokusho et al. (2013). We analyzed the X-ray spectra derived with the X-ray Imaging Spectrometer (XIS; Koyama et al., 2007), which covers an energy range of 0.5–12 keV. The XIS has two types of CCDs: FrontIlluminated (FI) CCDs (XIS 0 and XIS 3) and a Back-Illuminated (BI) CCD (XIS 1), and we merged the spectra of the two FI CCDs, which have almost the same response function, to improve the photon statistics. For data reduction and spectral analyses, we used the HEADAS software (version 6.14) and XSPEC software (version 12.8.1),
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T. Kokusho et al. / Planetary and Space Science 116 (2015) 92–96
Fig. 3. (a) A correlation plot between the intensities of the extinction-corrected [Fe II] 1.257 μm and the warm dust emission, shown together with the line with a slope of unity fitted to only the data points of Region A. Regions A–D are defined in Fig. 2. The data points are sampled every 1′ (130 pixels in the camera array). (b) The same as (a), but for the intensities of the extinction-corrected [Fe II] 1.257 μm and Paβ, shown together with the line with a slope of unity fitted to all the data points. (c) The same as (a), but for the warm dust emission newly derived in the present study (see text for details). (d) The extinction-corrected [Fe II] plotted against the warm and cold dust emission. The dotted vertical line shows the level of the foreground and background cold dust emission. Panel (a) is the same as Fig. 3 in Kokusho et al. (2013).
respectively. Suzaku observed northern and southern regions of IC 443, and the log of the observations is summarized in Table 1. Fig. 1 shows an X-ray (4.0–7.3 keV) spectrum of IC 443, observ ed with the Suzaku/XIS for a local area of 4′.4 × 4′.4 centered at (R. A., decl.) ¼(06h17m 29s , 22°47′33″). We gridded the northern and southern regions into areas of 4′.4 × 4′.4 and 5′.0 × 5′.0, respectively, which resulted in 67 local regions. Then we fitted the spectrum of each local region with the model consisting of a bremsstrahlung continuum and 6.7 keV Fe-K line emission to obtain the line intensity. In this fitting, we added an absorbed power-law component to the model as the cosmic X-ray background (Kushino et al., 2002). Finally, we created the intensity map of the 6.7 keV Fe-K line.
3. Result Fig. 2 displays the [Fe II] 1.257 μm and Paβ intensity maps. As can be seen in the figure, we detect both line emissions from a very wide area of the observed region, including the central and southern regions. This result confirms that fast shocks are propagating not only in the northeastern shell, as indicated by the 2MASS observation (Rho et al., 2001), but also in the inner and southern regions of IC 443. The figure also exhibits a global similarity in the spatial distribution between [Fe II] and Paβ. The contours in Fig. 2 show the distribution of the shock-heated warm dust emission associated with IC 443, which is obtained by fitting
the mid- to far-IR spectral energy distributions (SEDs) created by the AKARI and Spitzer data (Kokusho et al., 2013). Regions A–D are the same as defined in Kokusho et al. (2013). In order to derive the intrinsic intensities of both lines, we corrected the foreground extinction toward IC 443 by using the ratios of [Fe II] 1.257 μm to [Fe II] 1.644 μm as described in Kokusho et al. (2013). We also removed point sources with SExtracter (Bertin and Arnouts, 1996). Fig. 3a is the same plot as shown in Fig. 3 in Kokusho et al. (2013), while Fig. 3b shows a correlation plot between the intensities of the extinction-corrected [Fe II] 1.257 μm and Paβ. Fig. 3c and d shows plots similar to Fig. 3a, but newly derived in the present study. Kokusho et al. (2013) estimated shock-heated warm dust emission using a twotemperature modified blackbody model with the temperatures fixed. In the present study, we allow the warm dust temperature to vary (Fig. 3c) and add the cold dust component (Fig. 3d). In Fig. 3c and d, we confirm that the variable warm dust temperature does not significantly change the result in Fig. 3a, while the addition of the cold dust component changes the result. However the foreground and background cold dust emission is estimated to be 2.5 × 10−3 ergs s−1 cm−2 sr−1 from the 2′.5 × 2′.5 region centered at (R. A ., decl. ) = (06h17m 31s , 22°31′ 52″), and therefore most of the total dust emission in Regions B and C is not likely to be associated with IC 443. Since the [Fe II] emission is notably strong relative to the warm dust emission in Regions B and C, Kokusho et al. (2013) mentioned the possibility that Fe of ejecta origins is dominant in the central
T. Kokusho et al. / Planetary and Space Science 116 (2015) 92–96
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We fit the 4.0–7.3 keV X-ray spectrum in Region B by a plasma 1.0 model to obtain the Fe abundance of 0. 3−+0.2 solar, which suggests that the highly ionized Fe in this region is mostly of interstellar dust origins.
4. Discussion
B
C
Fig. 4. Intensity map of the 6.7 keV Fe-K line gridded into boxes of 4′.4 × 4′.4 and 5′.0 × 5′.0 for the northern and southern regions, respectively, which is shown together with the contour maps of the warm dust continuum (yellow) and the [Fe II] 1.257 μm intensity smoothed with a Gaussian kernel of σ = 45″ (blue). The color levels are given in units of photons s−1 cm−2 sr−1. The contours are drawn from 1.5 × 10−5 to 7.5 × 10−5 ergs s−1 cm−2 sr−1 on a linear scale for [Fe II], while they are the same as in Fig. 2 for the warm dust emission. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
region of IC 443. However Fig. 3b clearly shows that the [Fe II] intensity is globally correlated with the Paβ intensity in Regions A–D. This implies that relative abundances of singly ionized Fe to H are similar among the regions; Regions B and C show rather lower Fe+ abundances than Region A, which is in contrast with very high Fe þ abundance relative to dust in Regions B and C. Hence the result in Fig. 3a and b favors the possibility that Fe þ is mostly of interstellar origins not only in Regions A and D but also in Regions B and C, and thus the huge difference in the Fe þ to dust abundance ratio is likely to be attributed to difference in the degree of dust sputtering destruction. The lower abundance ratios of Fe þ to H in Regions B and C than in Region A may suggest further rapid ionization of Fe þ to higher degrees, as will be discussed later. In Fig. 3b, the ratios of [Fe II] 1.257 μm to Paβ are ∼2 in Regions A–D, while those in Orion are only 0.01 (Lowe et al., 1979), indicating that a large amount of gas-phase Fe þ is indeed released from dust through sputtering destruction in IC 443, while Fe is mostly depleted onto dust grains in Orion. Fig. 4 shows the intensity map of the X-ray 6.7 keV Fe-K line emission, with the contour maps of the warm dust continuum and the smoothed [Fe II] 1.257 μm emission. Although the X-ray observations do not cover the whole region of IC 443, the 6.7 keV Fe-K line emission is enhanced in the northwest region and extended toward Region C. The distribution of the He-like Fe ions seems to be spatially separated from that of the warm dust, in particular, near Region B. This indicates that, in the regions where X-ray plasma is dominant, Fe is ionized to a He-like state, while dust is almost completely destroyed. Toward Regions B and C, we significantly detect the 6.7 keV Fe-K line emission in addition to the [Fe II] emission, although there are significant offsets in the peak position between the [Fe II] and 6.7 keV Fe-K line emissions.
Dust in X-ray plasma is destroyed by sputtering on various timescales depending on plasma temperature, density, and dust chemical composition (e.g., Tielens et al., 1994; Jones et al., 1996). Kokusho et al. (2013) estimated the lifetime of dust in the X-ray plasma of IC 443 to be ∼1 × 105 yr , and the timescale for Fe to reach an ionization equilibrium in the same plasma to be ∼2 × 104 yr with the plasma temperature and density measured to be 107 K and 1.7 cm−3, respectively (Petre et al., 1988; Yamaguchi et al., 2009), and the dust size assumed to be 0.1 μm. According to these timescales, the dust lifetime is about 5 times longer than the ionization timescale of Fe, thus implying that most Fe ions should be in a He-like ionization state, but not singly ionized, in regions where dust is completely destroyed by plasma sputtering. Near the northeastern shell, which is rich in dust, Fe þ seems to be spatially separated from He-like Fe ions, which is quite reasonable considering the above short ionization timescale. However, as can be seen in Fig. 4, both [Fe II] and 6.7 keV Fe-K line emissions are detected toward Regions B and C where the dust emission is faint. How do we interpret the presence of abundant Fe in a singly ionized state in Regions B and C, the central part of IC 443 where the dust is likely to be destroyed almost completely? One possibility is a time delay in the release of gas-phase Fe from dust. If the interstellar grains are assumed to have Fe-rich cores with Fe-poor mantles, a substantial amount of Fe is released at the last stage of dust sputtering destruction while other species have already been released into a gas phase, which can explain the observed bright [Fe II] emission and faint dust emission. Another possibility is the pre-existing dense clumpy clouds that have prevented Fe from high ionization. In such clouds, a shock speed is decelerated so that plasma temperature may be too low to efficiently ionize Fe to a He-like state, but still high enough to destroy dust by sputtering. In order to discriminate the above two possibilities, more detailed analyses of the Suzaku X-ray data are needed. We will investigate the spatial distributions of metal abundances other than Fe and spectral variations of X-ray plasma temperature, the result of which will be reported in a future.
5. Conclusion We carried out line mapping of the whole structure of the Galactic SNR IC 443 with the IRSF/SIRIUS, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. As a result, we find that the [Fe II] and Paβ emissions are extended very widely not only in shell regions but also in inner regions, confirming that fast shocks are propagating in the entire remnant. The global distributions of the [Fe II] and Paβ intensities show a tight correlation between each other, implying that the relative abundance of Fe þ to H does not vary much across the observed regions. Kokusho et al. (2013) found that the [Fe II] emission is considerably strong relative to the shock-heated warm dust emission in the inner region of IC 443. These results indicate that the abundance of Fe þ is enhanced due to release of gas-phase Fe from dust by sputtering destruction and that the dust is almost completely destroyed. From the inner regions where the [Fe II] emission is detected, however, we also detect strong 6.7 keV Fe-K line emission with the Suzaku X-ray observation, implying that Fe has
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interacted with X-ray plasma for a time long enough to be highly ionized. In order to explain the detection of the Fe þ and He-like Fe ions toward the regions where the dust is almost absent, despite the short timescale for ionization equilibrium, we suggest the following two possibilities: a time delay in the release of gasphase Fe from dust and the pre-existing dense clumpy clouds which have prevented Fe from high ionization due to deceleration of shocks. The former situation can be realized by assuming that dust grains have Fe-rich cores with Fe-poor mantles and release a considerable amount of Fe into the interstellar space at the late stage of dust sputtering destruction.
Acknowledgments The IRSF project was financially supported by the Sumitomo foundation and Grants-in-Aid for Scientific Research on Priority Areas (A) (Nos. 10147207 and 10147214) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The operation of IRSF is supported by Joint Development Research of National Astronomical Observatory of Japan, and Optical NearInfrared Astronomy Inter-University Cooperation Program, funded by the MEXT of Japan. This research is based on observations with AKARI, a JAXA project with the participation of ESA, and with Spitzer, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research has made use of data obtained from the Suzaku satellite, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). T.K. is financially supported by Grantsin-Aid for JSPS Fellows No. 26003136.
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Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
A study of iron and dust in the supernova remnant IC 443 Takuma Kokusho a,n, Takahiro Nagayama b, Hidehiro Kaneda a, Daisuke Ishihara a, Ho-Gyu Lee c, Takashi Onaka d a
Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan Department of Astrophysics, Kagoshima University, Kagoshima 890-0065, Japan c Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea d Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2014 Received in revised form 19 February 2015 Accepted 16 April 2015 Available online 24 April 2015
We performed near-infrared line mapping of the supernova remnant IC 443 with the IRSF 1.4-m telescope, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. We detected the [Fe II] and Paβ emissions from a very wide area of the observed region (30′ × 35′). These line intensity maps reveal a global correlation between [Fe II] and Paβ. Kokusho et al. (2013) found that the [Fe II] emission is notably strong while dust emission is faint in inner regions of IC 443. These results indicate that the abundant Fe þ is likely to be of interstellar origins through shock destruction of dust, rather than of ejecta origins. From the X-ray intensity map of the 6.7 keV Fe-K line obtained with Suzaku, we find that highly ionized He-like Fe ions exist in the inner regions of IC 443, toward which Fe þ is detected. The presence of the He-like Fe ions and the faint dust emission indicates that Fe is likely to be interacting with X-ray plasma for a time long enough to be highly ionized. We discuss the implications of the detection of the lowly ionized Fe ions in such regions for the processing and composition of dust in the interstellar environment around IC 443. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Supernova remnant Interstellar dust Iron IRSF AKARI Suzaku
1. Introduction In supernova remnants (SNRs), the interstellar medium (ISM), composed of gas and dust, is heated and processed through shocks generated by supernovae, while supernovae also supply metal-rich ejecta synthesized in progenitors (e.g., Hughes et al., 2000), and dust into the interstellar space (e.g., Matsuura et al., 2011). In general, it is difficult to identify the elements depleted onto dust grains. However, in SNR shock regions, such elements incorporated in dust are released into the interstellar space through sputtering destruction of dust by X-ray plasma. Gas-phase elements thus liberated from dust can be excited in shock regions and emit spectral lines. In this way, SNRs give us an opportunity to investigate processing mechanisms and chemical compositions of dust. Fe, one of the abundant metals in the interstellar space, is thought to be heavily depleted onto dust grains (e.g., Draine, 1995). Since there is little direct evidence for the presence of Fe in dust, it is not well understood how Fe is contained in dust. For instance, it is suggested that silicate grains are generally poor in Fe (e.g., Henning, 2010), while McDonald et al. (2010) proposed that metallic Fe is formed in stellar winds. Pure Fe grains, if any, are n
Corresponding author. E-mail address: [email protected] (T. Kokusho).
http://dx.doi.org/10.1016/j.pss.2015.04.009 0032-0633/& 2015 Elsevier Ltd. All rights reserved.
difficult to identify, because they exhibit a featureless spectrum (e.g., Rho et al., 2008). In order to address such an Fe problem, SNRs, where shock-heated plasmas are sputtering away the elements contained in dust, offer valuable probes to examine forms and amounts of Fe in dust. IC 443 is one of the Galactic SNRs where shocks are strongly interacting with the surrounding ISM. The SNR is located in the direction of the Galactic anti-center at a distance of 1.5 kpc (Welsh and Sallmen, 2003), and its age is estimated to be in the range of 3000–30,000 years (Petre et al., 1988; Olbert et al., 2001). A pulsar wind nebula and metal-rich X-ray plasma are detected in the southern and northern regions of IC 443, respectively (Gaensler et al., 2006; Troja et al., 2008), indicating that the SNR is of a corecollapse origin. From limb-brightened morphologies at optical, infrared, and radio wavelengths (Bykov et al., 2008; Castelletti et al., 2011) and a centrally peaked morphology in the X-ray (Troja et al., 2008), IC 443 is categorized as a mixed-morphology-type SNR. In relation to the interaction between the shocks and the surrounding ISM in IC 443, an outstanding feature is seen in the nearinfrared (IR) wavelength. With the Two Micron All Sky Survey (2MASS), Rho et al. (2001) revealed that ionic and molecular shocks are propagating in IC 443. The northeastern shell of IC 443 is bright in the J and H bands, while it is faint in the Ks band. In this region, Graham et al. (1987) detected the [Fe II] 1.644 μm and Brγ emissions, who found that the ratio of [Fe II] 1.644 μm to Brγ is
T. Kokusho et al. / Planetary and Space Science 116 (2015) 92–96
considerably larger than that expected for HII regions, indicating a significant increase in the amount of gas-phase Fe due to the dust destruction. For the northeastern shell, Rho et al. (2001) concluded that the J band and the H band are dominated by the [Fe II] 1.257 μm and Paβ, and the [Fe II] 1.644 μm emissions, respectively. In contrast to the northeast region, the southern shell is bright only in the Ks band. With the near-IR spectroscopy, strong H2 line emission is detected from parts of the southern ridge (e.g., Richter et al., 1995; Shinn et al., 2011), and therefore, H2 seems to be a dominant career in the Ks band. In this region, Noriega-Crespo et al. (2009) detected the [Fe II] 26 μm emission with the Spitzer/ Infrared Spectrograph (IRS), which showed that faint [Fe II] filaments, such as not recognized with 2MASS, exist also in the southern region. To destroy dust and release Fe from dust into the interstellar space, fast shocks ( ≥ 150 km s−1) are needed (Jones et al., 1994), while H2 is dissociated by such fast shocks. Therefore, from the above near-IR morphology of IC 443, fast shocks are dominant in the northern shell, while shocks seem to be decelerated by the dense ISM in the southern ridge, where molecular clouds are distributed (e.g., Lee et al., 2012). In this paper, in order to study Fe and dust in IC 443, we focus on the spatial distributions of [Fe II] and Paβ in the near-IR, and X-ray 6.7 keV Fe-K (He-like ion) line emission as well as mid- and far-IR dust continuum emission in IC 443. To investigate the spatial distributions of the [Fe II] and Paβ emissions, we performed line mapping of IC 443 by a ground-based telescope. Comparing these maps with those in dust continuum and 6.7 keV Fe-K line emission obtained by space telescopes, we examine dust processing and composition through shock destruction in the SNR. Table 1 Log of the Suzaku observations. Observation ID
Date
(R.A., decl.)J2000
Exposure time (k sec)
501006020 507015020 507015030 507015040
7 March 2007 27 March 2013 31 March 2013 6 April 2013
(94.2972, (94.3028, (94.3024, (94.3024,
44 59 131 76
22.4797) 22.7465) 22.7479) 22.7479)
Counts/s/keV
2. Observation and data reduction
pixel scale of the SIRIUS camera are 7′.7 × 7′.7 and 0″.45, respectively. In order to investigate the main shell structure of IC 443, we observed 44 overlapped field of views, covering a large area of
10−3
10−4 4 2 χ
Fig. 2. Pseudo-color image of IC 443 (blue: [Fe II] 1.257 μm, red: Paβ, both for a range of 1.0 × 10−5–2.0 × 10−4 ergs s−1 cm−2 sr−1), where point sources are removed and the contour map of the warm dust emission is superimposed with the levels of 0.5, 1.5, 2.5, and 3.5 × 10−3 ergs s−1 cm−2 sr−1. The color images are smoothed with a Gaussian kernel of σ = 2″.3. The original data for the [Fe II] 1.257 μm and the warm dust emission are taken from Kokusho et al. (2013). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
We carried out line mapping of IC 443 with the near-IR camera SIRIUS (Simultaneous InfraRed Imager for UnbiasedSurvey; Nagashima et al., 1999; Nagayama et al., 2003) on the IRSF (InfraRed Survey Facility) 1.4-m telescope, located at the South African Astronomical Observatory, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. A field of view and a
0.01
0 −2 −4
93
4.5
5
5.5
6
6.5
7
Energy (keV) Fig. 1. XIS spectrum detected with the Suzaku/XIS from a local area (4′.4 × 4′.4 ) at (R. A ., decl. ) = (06h17m 29s , 22°47′33″), and the result of spectral fitting for an energy range of 4.0–7.3 keV (reduced χ 2 = 0.87). The top panel shows the best-fit model composed of bremsstrahlung continuum, 6.7 keV Fe-K line, and absorbed power-law (the cosmic X-ray background) components overlaid on the spectrum with the error bar of 1s. The bottom panel shows the residuals between the data and fitted model.
30′ × 35′. The details of the observation for the [Fe II] emission are described in Kokusho et al. (2013). We observed the Paβ emission in November 2012 and February 2013, with exposure time of 60 s and the dithering number of 10. For data reduction, we applied the same standard procedure as explained in Kokusho et al. (2013). For the flux calibration of the Paβ intensity map, we used the J-band data, assuming that the magnitudes of stars are the same between the narrow- and broad-band images. We analyzed the mid- and far-IR data obtained by AKARI and Spitzer, and the X-ray data by Suzaku. The details of the AKARI observation and data reduction are described in Kokusho et al. (2013). We analyzed the X-ray spectra derived with the X-ray Imaging Spectrometer (XIS; Koyama et al., 2007), which covers an energy range of 0.5–12 keV. The XIS has two types of CCDs: FrontIlluminated (FI) CCDs (XIS 0 and XIS 3) and a Back-Illuminated (BI) CCD (XIS 1), and we merged the spectra of the two FI CCDs, which have almost the same response function, to improve the photon statistics. For data reduction and spectral analyses, we used the HEADAS software (version 6.14) and XSPEC software (version 12.8.1),
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Fig. 3. (a) A correlation plot between the intensities of the extinction-corrected [Fe II] 1.257 μm and the warm dust emission, shown together with the line with a slope of unity fitted to only the data points of Region A. Regions A–D are defined in Fig. 2. The data points are sampled every 1′ (130 pixels in the camera array). (b) The same as (a), but for the intensities of the extinction-corrected [Fe II] 1.257 μm and Paβ, shown together with the line with a slope of unity fitted to all the data points. (c) The same as (a), but for the warm dust emission newly derived in the present study (see text for details). (d) The extinction-corrected [Fe II] plotted against the warm and cold dust emission. The dotted vertical line shows the level of the foreground and background cold dust emission. Panel (a) is the same as Fig. 3 in Kokusho et al. (2013).
respectively. Suzaku observed northern and southern regions of IC 443, and the log of the observations is summarized in Table 1. Fig. 1 shows an X-ray (4.0–7.3 keV) spectrum of IC 443, observ ed with the Suzaku/XIS for a local area of 4′.4 × 4′.4 centered at (R. A., decl.) ¼(06h17m 29s , 22°47′33″). We gridded the northern and southern regions into areas of 4′.4 × 4′.4 and 5′.0 × 5′.0, respectively, which resulted in 67 local regions. Then we fitted the spectrum of each local region with the model consisting of a bremsstrahlung continuum and 6.7 keV Fe-K line emission to obtain the line intensity. In this fitting, we added an absorbed power-law component to the model as the cosmic X-ray background (Kushino et al., 2002). Finally, we created the intensity map of the 6.7 keV Fe-K line.
3. Result Fig. 2 displays the [Fe II] 1.257 μm and Paβ intensity maps. As can be seen in the figure, we detect both line emissions from a very wide area of the observed region, including the central and southern regions. This result confirms that fast shocks are propagating not only in the northeastern shell, as indicated by the 2MASS observation (Rho et al., 2001), but also in the inner and southern regions of IC 443. The figure also exhibits a global similarity in the spatial distribution between [Fe II] and Paβ. The contours in Fig. 2 show the distribution of the shock-heated warm dust emission associated with IC 443, which is obtained by fitting
the mid- to far-IR spectral energy distributions (SEDs) created by the AKARI and Spitzer data (Kokusho et al., 2013). Regions A–D are the same as defined in Kokusho et al. (2013). In order to derive the intrinsic intensities of both lines, we corrected the foreground extinction toward IC 443 by using the ratios of [Fe II] 1.257 μm to [Fe II] 1.644 μm as described in Kokusho et al. (2013). We also removed point sources with SExtracter (Bertin and Arnouts, 1996). Fig. 3a is the same plot as shown in Fig. 3 in Kokusho et al. (2013), while Fig. 3b shows a correlation plot between the intensities of the extinction-corrected [Fe II] 1.257 μm and Paβ. Fig. 3c and d shows plots similar to Fig. 3a, but newly derived in the present study. Kokusho et al. (2013) estimated shock-heated warm dust emission using a twotemperature modified blackbody model with the temperatures fixed. In the present study, we allow the warm dust temperature to vary (Fig. 3c) and add the cold dust component (Fig. 3d). In Fig. 3c and d, we confirm that the variable warm dust temperature does not significantly change the result in Fig. 3a, while the addition of the cold dust component changes the result. However the foreground and background cold dust emission is estimated to be 2.5 × 10−3 ergs s−1 cm−2 sr−1 from the 2′.5 × 2′.5 region centered at (R. A ., decl. ) = (06h17m 31s , 22°31′ 52″), and therefore most of the total dust emission in Regions B and C is not likely to be associated with IC 443. Since the [Fe II] emission is notably strong relative to the warm dust emission in Regions B and C, Kokusho et al. (2013) mentioned the possibility that Fe of ejecta origins is dominant in the central
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We fit the 4.0–7.3 keV X-ray spectrum in Region B by a plasma 1.0 model to obtain the Fe abundance of 0. 3−+0.2 solar, which suggests that the highly ionized Fe in this region is mostly of interstellar dust origins.
4. Discussion
B
C
Fig. 4. Intensity map of the 6.7 keV Fe-K line gridded into boxes of 4′.4 × 4′.4 and 5′.0 × 5′.0 for the northern and southern regions, respectively, which is shown together with the contour maps of the warm dust continuum (yellow) and the [Fe II] 1.257 μm intensity smoothed with a Gaussian kernel of σ = 45″ (blue). The color levels are given in units of photons s−1 cm−2 sr−1. The contours are drawn from 1.5 × 10−5 to 7.5 × 10−5 ergs s−1 cm−2 sr−1 on a linear scale for [Fe II], while they are the same as in Fig. 2 for the warm dust emission. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
region of IC 443. However Fig. 3b clearly shows that the [Fe II] intensity is globally correlated with the Paβ intensity in Regions A–D. This implies that relative abundances of singly ionized Fe to H are similar among the regions; Regions B and C show rather lower Fe+ abundances than Region A, which is in contrast with very high Fe þ abundance relative to dust in Regions B and C. Hence the result in Fig. 3a and b favors the possibility that Fe þ is mostly of interstellar origins not only in Regions A and D but also in Regions B and C, and thus the huge difference in the Fe þ to dust abundance ratio is likely to be attributed to difference in the degree of dust sputtering destruction. The lower abundance ratios of Fe þ to H in Regions B and C than in Region A may suggest further rapid ionization of Fe þ to higher degrees, as will be discussed later. In Fig. 3b, the ratios of [Fe II] 1.257 μm to Paβ are ∼2 in Regions A–D, while those in Orion are only 0.01 (Lowe et al., 1979), indicating that a large amount of gas-phase Fe þ is indeed released from dust through sputtering destruction in IC 443, while Fe is mostly depleted onto dust grains in Orion. Fig. 4 shows the intensity map of the X-ray 6.7 keV Fe-K line emission, with the contour maps of the warm dust continuum and the smoothed [Fe II] 1.257 μm emission. Although the X-ray observations do not cover the whole region of IC 443, the 6.7 keV Fe-K line emission is enhanced in the northwest region and extended toward Region C. The distribution of the He-like Fe ions seems to be spatially separated from that of the warm dust, in particular, near Region B. This indicates that, in the regions where X-ray plasma is dominant, Fe is ionized to a He-like state, while dust is almost completely destroyed. Toward Regions B and C, we significantly detect the 6.7 keV Fe-K line emission in addition to the [Fe II] emission, although there are significant offsets in the peak position between the [Fe II] and 6.7 keV Fe-K line emissions.
Dust in X-ray plasma is destroyed by sputtering on various timescales depending on plasma temperature, density, and dust chemical composition (e.g., Tielens et al., 1994; Jones et al., 1996). Kokusho et al. (2013) estimated the lifetime of dust in the X-ray plasma of IC 443 to be ∼1 × 105 yr , and the timescale for Fe to reach an ionization equilibrium in the same plasma to be ∼2 × 104 yr with the plasma temperature and density measured to be 107 K and 1.7 cm−3, respectively (Petre et al., 1988; Yamaguchi et al., 2009), and the dust size assumed to be 0.1 μm. According to these timescales, the dust lifetime is about 5 times longer than the ionization timescale of Fe, thus implying that most Fe ions should be in a He-like ionization state, but not singly ionized, in regions where dust is completely destroyed by plasma sputtering. Near the northeastern shell, which is rich in dust, Fe þ seems to be spatially separated from He-like Fe ions, which is quite reasonable considering the above short ionization timescale. However, as can be seen in Fig. 4, both [Fe II] and 6.7 keV Fe-K line emissions are detected toward Regions B and C where the dust emission is faint. How do we interpret the presence of abundant Fe in a singly ionized state in Regions B and C, the central part of IC 443 where the dust is likely to be destroyed almost completely? One possibility is a time delay in the release of gas-phase Fe from dust. If the interstellar grains are assumed to have Fe-rich cores with Fe-poor mantles, a substantial amount of Fe is released at the last stage of dust sputtering destruction while other species have already been released into a gas phase, which can explain the observed bright [Fe II] emission and faint dust emission. Another possibility is the pre-existing dense clumpy clouds that have prevented Fe from high ionization. In such clouds, a shock speed is decelerated so that plasma temperature may be too low to efficiently ionize Fe to a He-like state, but still high enough to destroy dust by sputtering. In order to discriminate the above two possibilities, more detailed analyses of the Suzaku X-ray data are needed. We will investigate the spatial distributions of metal abundances other than Fe and spectral variations of X-ray plasma temperature, the result of which will be reported in a future.
5. Conclusion We carried out line mapping of the whole structure of the Galactic SNR IC 443 with the IRSF/SIRIUS, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. As a result, we find that the [Fe II] and Paβ emissions are extended very widely not only in shell regions but also in inner regions, confirming that fast shocks are propagating in the entire remnant. The global distributions of the [Fe II] and Paβ intensities show a tight correlation between each other, implying that the relative abundance of Fe þ to H does not vary much across the observed regions. Kokusho et al. (2013) found that the [Fe II] emission is considerably strong relative to the shock-heated warm dust emission in the inner region of IC 443. These results indicate that the abundance of Fe þ is enhanced due to release of gas-phase Fe from dust by sputtering destruction and that the dust is almost completely destroyed. From the inner regions where the [Fe II] emission is detected, however, we also detect strong 6.7 keV Fe-K line emission with the Suzaku X-ray observation, implying that Fe has
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interacted with X-ray plasma for a time long enough to be highly ionized. In order to explain the detection of the Fe þ and He-like Fe ions toward the regions where the dust is almost absent, despite the short timescale for ionization equilibrium, we suggest the following two possibilities: a time delay in the release of gasphase Fe from dust and the pre-existing dense clumpy clouds which have prevented Fe from high ionization due to deceleration of shocks. The former situation can be realized by assuming that dust grains have Fe-rich cores with Fe-poor mantles and release a considerable amount of Fe into the interstellar space at the late stage of dust sputtering destruction.
Acknowledgments The IRSF project was financially supported by the Sumitomo foundation and Grants-in-Aid for Scientific Research on Priority Areas (A) (Nos. 10147207 and 10147214) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The operation of IRSF is supported by Joint Development Research of National Astronomical Observatory of Japan, and Optical NearInfrared Astronomy Inter-University Cooperation Program, funded by the MEXT of Japan. This research is based on observations with AKARI, a JAXA project with the participation of ESA, and with Spitzer, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research has made use of data obtained from the Suzaku satellite, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). T.K. is financially supported by Grantsin-Aid for JSPS Fellows No. 26003136.
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