CALCIUM-RICH GAP TRANSIENTS: SOLVING THE CALCIUM CONUNDRUM IN THE INTRACLUSTER MEDIUM

The intracluster medium (ICM) contains the majority of the baryons in clusters of galaxies. The presence of heavy elements in the ICM indicates that a substantial fraction of the diffuse gas must have passed through stars. Although the presence of iron in the ICM has been known about for decades (Mitchell et al. 1976; Serlemitsos et al. 1977; Mushotzky et al. 1978), observations with ASCA, Chandra, XMM-Newton, and Suzaku have provided constraints on many additional elements. In particular, the abundances of O, Ne, Mg, Si, S, Ca, Ar, Ni, and Fe have now been measured in many nearby clusters (Mushotzky et al. 1996; Fukazawa et al. 1998; Finoguenov et al. 2000; de Plaa et al. 2007; Werner et al. 2008; Bulbul et al. 2012).

Like all metals in diffuse gas, the metals in the ICM originate primarily from supernova explosions. Therefore, X-ray measurements of the composition of the ICM provide a means of studying the history of supernovae over the lifetime of a cluster. Many authors have used abundance measurements to constrain the relative contribution of Type Ia and core-collapse supernovae to the enrichment of the ICM. The abundance pattern of most clusters appears to require a ∼30%–50% contribution from Type Ia supernovae. While a combination of Type Ia and core-collapse yields can explain the observed abundances of most elements, recent work suggests that such models underproduce the amount of calcium required by X-ray observations (Werner et al. 2008; de Plaa et al. 2007). Several solutions for the calcium problem have been suggested in the literature including the possible underestimate of calcium yields in core-collapse models (Werner et al. 2008) or the modification of Type Ia calcium yields based on measurements from the Tycho supernova remnant or changing the point at which deflagration transitions to detonation (de Plaa et al. 2007).

Most of the previous attempts to model the abundances of the ICM have made the simplified assumption of uniform yields for Type Ia and core-collapse supernovae. However, extensive supernova searches over the last decade have revealed considerable diversity among supernovae and likely a wide range in yields. Here, we investigate whether including a contribution from a new class of transients that have nebular spectra dominated by calcium can help explain the calcium abundance in the ICM.

The existence of transients with spectra dominated by calcium was first reported by Filippenko et al. (2003) based on observations from the Lick Observatory Supernova Search (Li et al. 2011). Perets et al. (2010) performed the first detailed study of a member of this class (SN 2005E). Subsequent examples of calcium-rich supernovae have been found by both the Palomar Transient Factory (Kasliwal et al. 2012) and the Catalina Real Time Survey (Valenti et al. 2013). Modeling the nebular spectra shows that the total ejected mass of SN 2005E was very low (M_ej ∼ 0.3 M_☉); nearly 50% of the ejected mass is in calcium (Perets et al. 2010). The total calcium synthesized by SN 2005E is therefore a factor of 5–10 greater than that of normal Type I supernovae.

Although only a small number of calcium-rich supernovae have been identified so far, a strong case can be made for this representing a distinctive class of objects. All members have five distinguishing characteristics: (1) peak luminosity in the gap between classic novae and supernovae, (2) rapid photometric evolution, (3) large photospheric velocities, (4) early spectroscopic evolution into the nebular phase, and (5) nebular spectra dominated by calcium (Kasliwal et al. 2012). As the peak luminosity of these physically different explosions falls in between that of novae and classic supernovae, these objects are referred to as calcium-rich gap transients. In addition to the characteristics given above, these events appear to occur mostly in the outskirts of their hosts or in intragroup/intracluster regions, strongly suggesting an explosion in an old population of compact binaries. Yuan et al. (2013) suggest that the locations of calcium-rich gap transients may be consistent with globular clusters. Lyman et al. (2013) show that there is no sign of in situ star formation. No model can yet explain all the characteristics of this class.

In order to understand whether the large calcium yields of the calcium-rich gap transients can help explain the high abundance of calcium seen in the ICM, we examine a series of supernova models that includes contributions from core-collapse, Type Ia, and calcium-rich gap transients. We compare these models to the observed ICM abundances following the procedure outlined in de Plaa et al. (2007). These ICM measurements have the advantage of being derived from a sample of 22 clusters, while most other studies have considered one cluster at a time. The de Plaa et al. (2007) data set provides measurements for the elements of Si, S, Ar, Ca, Fe, and Ni.

For each of the six elements under consideration, we perform a simple linear fit to the observed abundances:

$$\begin{equation} {\eta _{{\rm tot}}} \sum _i \alpha _i N_{i,j} = X_{j}, \end{equation} \tag{ 1 }$$

where η_tot is the total number of supernovae, α_i is the fitted fraction for each of three supernova types, N_{i, j} is the predicted yield of an element per supernova, and X_j is the X-ray-derived intracluster abundance for each of six elements. We compute N_{i, j} from mass yields as follows:

$$\begin{equation} N_{i,j} = \frac{Y_{i,j}}{\mu _{j}\xi _{j}}, \end{equation} \tag{ 2 }$$

where Y_{i, j} is the predicted mass yield for each element per supernova, μ_j is the mean atomic mass, and ξ_j is the protosolar elemental abundance taken from Lodders (2003). We adopt the yields given in Nomoto et al. (2006) for the core-collapse supernovae. Unfortunately, there has been very little theoretical work on the potential yields of calcium-rich gap transients. Therefore, we adopt the yields given in Perets et al. (2010) for SN 2005E for this entire class of calcium-rich supernovae.

For the Type Ia yields, we first consider the WDD2 model given in Iwamoto et al. (1999) as this has previously been shown to provide the best fits to the X-ray data (de Plaa et al. 2007). Figure 1 shows the result of taking the best-fit supernova rates given in de Plaa et al. (2007), but with 10% of the Type Ia supernovae assumed to be calcium-rich gap transients. As can be seen from the figure, including the calcium-rich gap transients in the fit improves the fit (χ² of 21 compared to 84) for the calcium abundance while having little effect on the other elements. Assuming the raw volumetric relative rates of supernovae improves the χ² to 40 (see Table 2).

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If we allow the rates of each supernova type to vary, we improve the χ² to 2.53. Figure 2 shows this best-fit with the WDD2 model. The best-fit rate of calcium-rich gap transients is 16% of the rate of Type Ia supernovae. We note that volumetric rate estimates for calcium-rich gap transients relative to Type Ia supernovae vary due to the unknown luminosity function correction and small sample size. Some recent estimates are 7% ± 5% (Perets et al. 2010), <20% (Li et al. 2011), and >2.3% from the Palomar Transient Factory (Kasliwal et al. 2012). Therefore, the best fit rate of 16% is consistent with current limits on rates from observational searches for these transients. We caution that this best-fit rate is degenerate with the calcium yield per Ca-rich gap transient. Thus, properly accounting for this subclass of supernovae in the modeling of the ICM abundances may indeed provide a solution to the "calcium conundrum" in clusters.

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Next, we consider several different models for Type Ia supernovae (see Table 1) and allow the relative fraction of each supernova type to vary. In Table 2 we show the relative rates and derived χ² for each of the Type Ia models considered that yielded reasonable fits. All models yield a χ² that is significantly better than that in previous work that did not take into account calcium-rich gap transients. These models have a varying degree of success in explaining the observed abundance pattern. With the inclusion of a contribution from calcium-rich gap transients, all of the models reasonably fit the lower atomic number elements (see Figure 3). The poor χ² values for most models are due entirely to the over- or underproduction of nickel. Only the WDD2 and O-DDT models are close to reproducing the observed abundance of this element. It is worth noting that for these two models the best-fit solutions require a higher rate of Type Ia supernovae relative to core-collapse supernovae than is observed in the local volume surveys found by other authors.

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In Section 3, we showed that simply including a contribution from the recently discovered calcium-rich gap transients can potentially explain the high calcium abundances reported in the ICM. Here we explore the implications of this conclusion and what it might mean for the enrichment process in clusters of galaxies.

Although the number of calcium-rich gap transients studied so far is small, they appear to reside in locations atypical of supernovae (Perets et al. 2010; Kasliwal et al. 2012; Valenti et al. 2013). Examples have now been found in elliptical galaxies, above the disks of edge-on galaxies, and at large distances from isolated hosts. Moreover, all of the known examples appear to be associated with groups or clusters of galaxies. The remote locations of these events suggest that these supernovae may preferentially occur in regions of low metallicity. The off-galaxy locations may also work to maximize the contribution of calcium-rich gap transients to the enrichment of the ICM in galaxy clusters. For example, the metals from calcium-rich events in the outskirts of galaxies would be more easily stripped via ram pressure as galaxies fall into the cluster than gas concentrated near the galaxy center. Similarly, calcium produced in small groups of galaxies could easily be mixed in with the ICM when these systems get incorporated into bigger systems.

Given the large population of intracluster stars found in clusters, some calcium-rich supernovae may also occur in situ. Supernovae with no host galaxy have indeed been found in studies of nearby clusters (Gal-Yam et al. 2003; Dilday et al. 2010; Sharon et al. 2010; Sand et al. 2011). Such supernovae may be very efficient at polluting the ICM as their metals can be directly deposited into the cluster without needing to escape the confining potential of a galaxy (Gal-Yam & Maoz 2000; Zaritsky et al. 2004; Sivanandam et al. 2009; Rasmussen et al. 2010). The recent discovery of a calcium-rich gap transient in the Coma cluster far away from any galaxies (M. M. Kasliwal et al. 2013, in preparation) supports the idea that some calcium-rich gap transients are indeed associated with intracluster stars. Future transient surveys should provide constraints on the importance of intracluster calcium-rich gap transients.

Calcium-rich gap transients may also play an important role in enriching gas in other astrophysical systems. Although the uncertainties on the measurements are large, the calcium abundance of the intragroup medium in some groups of galaxies appears to be even higher than that in rich clusters (Grange et al. 2011). This might be expected given that most of the calcium-rich gap transients discovered to date occur in small groups and many of the stripping mechanisms at play in clusters are also effective in lower mass systems (Rasmussen et al. 2006; Kawata & Mulchaey 2008). The possible association of calcium-rich gap transients with low metallicity regions suggests that these transients may also be an importance source of enrichment in the halos of individual galaxies. This could help account for calcium-overabundant stars in the halo of the Milky Way (McWilliam et al. 1995; Lai et al. 2009) and the large amount of calcium in the circumgalactic medium (Zhu & Ménard 2013). Furthermore, the tendency for the calcium abundance to track iron rather than oxygen in early-type galaxies (Conroy et al. 2014) suggests most of the calcium in these galaxies is not produced by the core-collapse supernovae. Rather, the source of calcium is likely dominated by Type Ia and calcium-rich gap transients as we find for rich clusters (see Figure 2).

While our work suggests the potential importance of calcium-rich gap transients on the enrichment of the ICM, further study will be required to reach firm conclusions. For example, for our calculations we have relied on the yield estimates for a single supernova for the entire class of calcium-rich gap transients. High-quality nebular spectra are now available for several calcium-rich gap transients and more detailed theoretical modeling of these spectra would provide a more representative average yield of calcium per transient. Second, as with previous work, the core-collapse yields here include the contribution of both Type II and Type Ibc supernovae. Both the rates and the yields of each of these sub-types is different and it may be more accurate to consider their contributions separately. Third, the yields as determined by theoretical methods and nebular spectrum modeling methods are not consistent for Type Ia supernovae. More work must be done to understand these differences and their impact on the yield estimates. Finally, with the recent proliferation of all-sky transient surveys, multiple new classes of elusive explosions have been uncovered (see, e.g., brief review in Kasliwal 2012 and references therein). Yield estimates for each of these new classes should be undertaken to assess their contribution to the ICM. While exploring these effects is beyond the scope of this Letter, we hope that our simple calculations will provide motivation for further theoretical modeling for all supernova types.

The need for more detailed supernova modeling is further justified by upcoming X-ray missions like ASTRO-H (Takahashi et al. 2010), which will enable spatially resolved high-resolution X-ray spectroscopy of clusters for the first time. Detailed maps of the spatial variation of elements may provide important clues about the stars responsible for enriching the ICM.

We are grateful to Jelle de Plaa and Esra Bulbul for providing additional insight on their previous work and we thank Avishay Gal-Yam, Melissa Graham, and Paolo Mazzali for helpful discussions. This work was inspired by presentations at the conference Energetic Astronomy: Richard Mushotzky at 65. We thank Chris Reynolds for his help organizing the conference.

Facilities: XMM - Newton X-Ray Multimirror Mission satellite, CXO - Chandra X-ray Observatory satellite