Intrinsic color diversity of nearby type Ia supernovae
Noriaki Arima, Mamoru Doi, Tomoki Morokuma, Naohiro Takanashi
aa r X i v : . [ a s t r o - ph . H E ] J a n Intrinsic color diversity of nearby type Iasupernovae
Noriaki A
RIMA , ∗ Mamoru D OI , Tomoki M
OROKUMA , and NaohiroT AKANASHI Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa,Mitaka, Tokyo 181-0015, Japan Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Research Center for the Early Universe, Graduate School of Science, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo,5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan Executive Management Program, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo113-8654, Japan ∗ E-mail: [email protected]
Received h reception date i ; Accepted h acception date i Abstract
It has been reported that the extinction law for Type Ia Supernovae (SNe Ia) may be differentfrom the one in the Milky Way, but the intrinsic color of SNe Ia and the dust extinction areobservationally mixed. In this study, we examine photometric properties of SNe Ia in thenearby universe ( z < ∼ . ) to investigate the SN Ia intrinsic color and the dust extinction. Wefocus on the Branch spectroscopic classification of 34 SNe Ia and morphological types of hostgalaxies. We carefully study their distribution of peak colors on the B − V , V − R color-colordiagram, as well as the color excess and absolute magnitude deviation from the stretch-colorrelation of the bluest SNe Ia. We find that SNe Ia which show the reddest color occur inearly-type spirals and the trend holds when divided into Branch sub-types. The dust extinctionbecomes close to the Milky-Way like extinction if we exclude some peculiar red Broad Line BL) sub-type SNe Ia. Furthermore, two of these red BLs occur in elliptical galaxies, less-dusty environment, suggesting intrinsic color diversity in BL sub-type SNe Ia.
Key words: supernovae: general — galaxies: general — dust, extinction
Type Ia supernovae (SNe Ia) have been used as distance indicators thanks to their high luminosity,which can be observed at cosmological distances ( M B ≈ − . mag). Luminosity can be standardizedafter correcting the empirical relation between luminosity and light-curve decline rate: a brighterSN Ia diminishes slowly (so-called Phillips-relation: Phillips 1993; Hamuy et al. 1996; Riess et al.1996; Phillips et al. 1999). Their use for cosmological studies provided us an observational evidencethat the expansion speed of the Universe is accelerating (Riess et al. 1998; Perlmutter et al. 1999).Near the epoch of maximum brightness, SNe Ia show strong silicon absorption lines, espe-cially the Si II λ but neither hydrogen nor helium lines are present (Filippenko 1997; Gal-Yam2017). SNe Ia are thought to originate from carbon-oxygen (C/O) white dwarfs in close binary sys-tems. Theoretically, two most popular SN Ia scenarios are the single-degenerate (SD) model andthe double-degenerate (DD) model. In the SD model, a white dwarf accretes material from a non-degenerate companion star until it reaches near the Chandrasekhar limiting mass ( M Ch ≈ . M ⊙ ) andexplodes (Whelan & I. Iben 1973; Nomoto 1982). In the DD model, two white dwarfs merge after los-ing orbital energy and angular momentum by gravitational waves (I. Iben & Tutukov 1984; Webbink1984). However, we still don’t have clear physical understandings of the evolutionary paths toSNe Ia and there are several other potential progenitor models (see reviews of Maoz & Mannucci2012; Maeda & Terada 2016 and also Livio & Mazzali 2018). In addition, the Phillips-relation lackscomplete physical understanding.Identifying the differences and diversity of SNe Ia is important for understanding their pro-genitor scenarios and improving their effectiveness as a cosmological tool. Recent cosmologicalmeasurements with SNe Ia are not only limited by the sample size, but also by the systematic uncer-tainties due to the lack of our understanding the variety and physical mechanisms of SNe Ia (Conleyet al. 2010; Betoule et al. 2014). Although observables that characterize each SN Ia such as colormay give us a clue, scatters in colors and dust properties surrounding SNe Ia prevent us to reduceuncertainties for precision cosmology (e.g., see figure 1 of Sullivan et al. 2011).It is also critical for SN Ia studies to understand the properties of interstellar and circumstellar2ust. In general, extinction by dust is described by a parameter R V : total-to-selective extinction ratiodefined by R V = A V /E ( B − V ) , which reflects the property of dust. Large value of R V means largegrain size of dust (c.f., Clayton et al. 2003). The mean value in our Milky Way is R V = 3 . (Cardelliet al. 1989). It has been reported by previous studies that the dust extinction for SNe Ia has smallervalue of R V than the mean value of Milky Way. For example, Nobili & Goobar (2008) and Kessleret al. (2009) reported R V = 1 . and R V = 2 . , respectively (see also table 1 of Cikota et al. (2016)for the summary of small R V reported until then). Assuming a constant R V , Folatelli et al. (2010)analyzed the optical–NIR colors of the nearby Carnegie Supernova Project (CSP: Contreras et al.2010) SN Ia sample and they obtained R V ≈ . , but they obtained R V = 3 . ± . when the extremelyred objects ( E ( B − V ) > ∼ were excluded. Mandel et al. (2011) later did a more sophisticatedanalysis and showed variations in R V , with the low-extinction events giving higher values of R V ∼ and high-extinction events giving R V ∼ . Burns et al. (2014) found a similar result. In recent studies,Stanishev et al. (2018) used optical-NIR light curves to derive R V ≃ . − . and Cikota et al. (2016)also favors small R V but they obtained different R V values for SNe Ia with different host morphology( R V = 2 . ± . for SNe Ia observed in Sab–Sbp galaxies, and R V = 1 . ± . for SNe Ia observedin Sbc–Scp galaxies). On the other hand, based on a spectral series, Sasdelli et al. (2016) showed R V to be consistent with the typical Milky Way value. Mandel et al. (2017) constructed host galaxy dustmodels for SNe Ia and the dust extinction they obtained ( R B = 3 . ± . R B = R V + 1 ) also agreeswith the Milky Way dust extinction.Over the years, observations provided that there are varieties in luminosity and spectral fea-tures of SNe Ia. For example, 1991T-like and 1991bg-like SNe are well-known prominent outliers.The 1991T-like SNe are more luminous ( > ∼ III ) in their spectra at early times (Filippenko et al. 1992; Phillips et al.1992). As opposed to it, the 1991bg-like SNe are less-luminous and show rapidly evolving light-curveand strong Ti II lines (Nugent et al. 1995; Mazzali et al. 1997). Diagnosing spectra around maximumbrightness has been used to investigate the origins of the diversity.In Branch et al. (2006), SNe Ia were assigned into four groups according to measurements ofthe equivalent width (EW) of two Si II absorption features at about 5750 Å and 6100 Å which areattributed to rest-frame λ and λ lines, respectively. The four groups are: core-normal (CN),broad-line (BL), cool (CL), and shallow-silicon (SS). The 1991T-like and the 1991bg-like SNe Ia areassigned into the extreme end of SS and CL, respectively. Using early phase SN Ia colors, Stritzingeret al. (2018) found that there are two distinct populations with different early color evolution in B − V ,and the two early blue/red events are correlated with the Branch spectroscopic groups.Another spectroscopic approach is dividing SNe Ia into two groups in terms of the expansion3elocity estimated from the absorption minimum of Si II λ line. Wang et al. (2009) found thathigh velocity (HV) SNe Ia show redder B − V colors at maximum brightness than normal velocity(NV) SNe Ia. Zheng et al. (2018) found that SNe Ia with higher velocities are inferred to be intrinsi-cally fainter than the NV SNe Ia and they confirm that HV SNe Ia are probably intrinsically differentfrom NV SNe Ia.Focusing on the differences in environment is another approach to study the diversity of SNeIa. It is found that host galaxy stellar mass correlates with SNe Ia luminosity: SNe Ia in massive hostgalaxies are intrinsically more luminous after light-curve/color corrections (Kelly et al. 2010; Sullivanet al. 2010; Childress et al. 2013; Pan et al. 2014; Betoule et al. 2014). The latest cosmological studyby Smith et al. (2020) found that SNe Ia in high-mass galaxies ( > M ⊙ ) are intrinsically moreluminous than their low-mass counterparts by . ± . mag. Correlations with host-galaxymetallicity have also been studied (e.g., Moreno-Raya et al. 2016). Theoretically, the metallicity ofthe SN Ia progenitors affects the strength of spectral features more in the UV wavelengths than inthe optical (Lentz et al. 2000; Walker et al. 2012; Miles et al. 2016). However, each model has adifferent effect on the strength and wavelength range. A recent study using SNe Ia with normal light-curve shapes found that there is no significant correlations between the UV-optical colors of SNe Iaand the host-galaxy metallicity (Brown & Crumpler 2020), which is in contrast to the findings of Panet al. (2020). The physical relation between host metallicity and SN Ia properties is not yet clear.For fitting a light-curve shape to standardize SN Ia luminosity, "MLCS2k2" (Jha et al. 2007),"SALT2" (Guy et al. 2007) and "SNooPy" (Burns et al. 2011; Burns et al. 2014) are methods oftenused. Each method parameterizes observed light curves and estimates host-galaxy dust extinction atthe same time. When we investigate the host-galaxy dust extinction, we should treat dust extinctioneffect and SN Ia intrinsic colors independently, and hence we should not assume any dust extinctionmodels. In contrast, Takanashi et al. (2017) (hereafter TAK17) simply parameterizes only light curveshapes and peak brightness. They analyzed multi-band light curves of SNe Ia from SDSS-II SNSurvey and their result suggests that there seems to be inherently different sub-types among SNeIa with different colors and extinction laws of host galaxy dust, which is consistent with previouslysuggested ideas (e.g., Mannucci 2005; Quimby et al. 2007).As we mentioned above, the diversity of SN Ia spectra, luminosity and colors may be corre-lated with host properties and also different types of dust extinction. In order to improve accuracy asdistance indicators for cosmological studies, understanding the diversity is very important. In TAK17,they only studied photometric properties. In this paper we extend the study of TAK17 combining withthe spectroscopic classification of SNe Ia defined by Branch et al. (2009). In Section 2, we describeour SNe Ia sample with host galaxy morphology used in this study. In Section 3, we apply the method4sed in TAK17 to analyze photometric SNe Ia data and show the results of nearby SNe Ia with spec-tral classification and host morphology. In Section 4, we discuss our results and we summarize ourfindings in Section 5. We use the photometric data obtained from Takanashi et al. 2008 (hereafter, TAK08). In TAK08, theycollected U − , B − , V − , R − and I -band photometry of 122 nearby ( z < . ) SNe Ia from publishedsources. Magnitudes are presented in the Vega system, and are all K − corrected. As we mentioned inSection 1, well-known light curve fitting methods fit observed SNe Ia light curves by parameterizingtheir light-curve shape, peak brightness, color and extinction at the same time. In TAK08 (and alsoin TAK17), unlike these methods, their original "Multi-band Stretch Method" simply characterizeslight-curve shapes and peak brightness corrected for Galactic dust extinction from Schlegel et al.(1998) without dust extinction correction in the host galaxies. This makes it possible to investigatethe diversity of SN Ia light curves directly.We use B − ,V − and R − band absolute magnitude and B − band stretch factor (Goldhaber et al.2001), which is a parameter describing a width of SN Ia light-curve shape. In TAK08, they adopt thevalues of cosmological parameters, H = 70 . − Mpc − , Ω M = 0 . , Ω Λ = 0 . from Spergelet al. (2007) for calculating absolute magnitude. We adopt these values in this paper. As described in section 1, Branch et al. (2006) divide SNe Ia into four groups based on equivalentwidths (=EWs) of the two Si II λ , λ lines (= EW(6100) and EW(5750)); Core Normal (CN),Broad Line (BL), Cool (CL) and Shallow Silicon (SS). In this paper, we use the term "sub-type" torefer to each Branch spectroscopic group. CN SNe Ia have typical EWs for both Si II lines. BL SNeIa have 6100 Å absorption that is broader and deeper than CN SNe Ia and that means they have largeEW(6100). For CL SNe Ia, the name "Cool" comes from their low temperature compared with othersub-types and they show relatively large EWs for both lines. On the other hand, SS SNe Ia haveshallow Si II absorption lines, which implies the ejecta is in high temperature. In Branch et al. (2009),they say that there is no distinct threshold of these two equivalent widths that classifies them into foursub-types.In this study, combining photometry from TAK08 with the Branch spectroscopic sub-types(Branch et al. 2009), we collect nearby ( . < z < . ) 34 SNe Ia (hereafter, referred to as5Branch sample"). The Branch sample consists of 9 CN, 10 BL, 3 CL and 12 SS SNe Ia. Althoughthere are slightly different spectroscopic classifications among Branch et al. (2009) and the otherpapers, we adopt the classification presented in Branch et al. (2009) to keep consistency. In Branchet al. (2009), most of the spectra taken ± days from maximum phase are used , while for expamle,in Blondin et al. (2012), spectra taken within ± days from maximum phase are used. The propertiesof our sample are summarized in table 1. ∆ M B and SN CE
In TAK17, they analyzed multi-band light curves of 328 SNe Ia observed by the Sloan Digital SkySurvey-II Supernova Survey (Sako et al. 2008; Sako et al. 2018). Equations (1) and (2) show therelations between the inverse B − band stretch factor s − B ) and B − , V − band absolute magnitude ofthe SDSS bluest SNe Ia, which they call stretch-magnitude relations. The color range of the bluestSNe Ia is − . ≤ ( M B − M V ) ≤ − . . They introduced two parameters: ∆ M B and SN CE (=SuperNova Color Excess). They define ∆ M B as the magnitude difference between B − band absolutemagnitude, M B and estimated absolute magnitude with the stretch-magnitude relation of the bluestSN Ia sample. Also, SN CE is defined by the difference between observed B − V color and intrinsiccolor from the stretch-color relation. The idea is based on an assumption that relations among a light-curve shape parameter, color and luminosity obey a unique relation for all SNe Ia. The relation canbe derived from the bluest SNe Ia, for which extinction may be negligible. In case of the bluest SNeIa, by definition, both ∆ M B and SN CE become close to zero. Non-zero ∆ M B and SN CE can beinterpreted as either dust extinction or intrinsically different luminosity and color from the bluest SNeIa. M B (bluest SNe Ia) = (1 . ± . × s − B ) − (20 . ± . (1) M V (bluest SNe Ia) = (1 . ± . × s − B ) − (20 . ± . (2)Using the equations (1) and (2), the ∆ M B and the SN CE are derived as equations (3) and (4) ∆ M B = m B − µ − (1 . × s − B ) − . (3) SN CE = ( m B − m V ) − (0 . × s − B ) − . (4), where m B and m V are stretch-corrected apparent B − band and V − band magnitudes at B − bandmaximum and µ is distance modulus. We derive the ∆ M B and SN CE using the relations. The listof ∆ M B and SN CE of the Branch sample is shown in table 2. Some of the Branch samples (SN 1997br, 1997do, 1998ab, 1999gh, 2000cn and 2001V) have no Si II EWs measurements within ± days from maximumphase. .4 Host galaxy morphology Because there are strong correlations between SNe Ia and their host galaxy properties (e.g., Sullivanet al. 2006), the inclusion of host galaxy morphology will give us implications to study the SN Iaintrinsic color diversity. We obtained the Hubble type of host galaxies from the NED (NASA/IPACExtragalactic Database) and then use their host type index T (de Vaucouleurs 1959) to divide SNeIa into different host morphological groups. We divide SNe Ia into three groups: early-type galaxies(E/S0; T ≤ ), early-type spirals (from Sa to Sb; ≤ T ≤ ) and late-type spirals or irregular galaxies(Sc or later and Irr; T ≥ ). In figure 1, we show a B − V vs V − R color-color diagram. Two different extinction laws with R V = 3 . and R V = 2 . are shown by black solid and red dashed lines respectively. Extinction lawwith larger R V has steeper slope in this diagram. In figure 1, the distribution seems to be moreconsistent with the extinction law with R V = 2 . than R V = 3 . for our sample. There are, however,apparently three BL objects (SN 1999cl, 1999gh and 2000B) that are out of the "main sequence" ofthe distribution. These three BLs are on the lower side of R V = 2 . line and this result is consistentwith previous studies that SNe Ia with high Si II λ expansion velocity have redder colors andprefer a lower R V (e.g., Wang et al. 2009; Foley & Kasen 2011). Note that though relatively largeEW(6100) labels SN 1999gh as BL, SN 1999gh also has a large EW(5750), which moves it up intothe CL sub-type region in the Branch diagram (see figure 2 of Branch et al. 2009). SN 2000B has noEW measurements in Branch et al. (2009). CL objects have intrinsically red colors because of theirlow temperature. If SN 1999gh and 2000B are actually CL sub-type, then the distribution is attributedmostly to intrinsic color rather than interstellar dust.Next we examine the color dependence on host morphology. In figure 2, different symbolsshow the different host morphological groups (circles; E/S0, pentagons; from Sa to Sb and stars;Sc or later and Irr). A trend can be seen that SNe Ia whose hosts are early-type spirals (pentagons)have the reddest color along with the dust extinction with R V = 2 . . This result implies that there isintrinsic color offset depending on the host galaxy morphology as it has been said in previous studies(e.g., Sullivan et al. 2010; Pan et al. 2014). The breakdown of the host morphological dependence isshown in figure 3. Although the sample size is not large, figure 3 shows the trend seen in figure 2,SNe Ia which occur in early-type spirals have the reddest color, holds even after divided into Branchsub-types. 7he effect of s ( B ) on the B − V and V − R colors is inferred from equations (1), (2) and the R − band regression of the bluest SNe Ia . In figure 2, the blue arrow shows the variations of colorsfor the bluest SNe Ia when s ( B ) varies from s ( B ) = 0 . to s ( B ) = 1 . . It is expected that colors for mostobjects will move towards the dust extinction lines. The two BL objects (SN 1999gh and 2000B)however, have small s ( B ) ( s ( B ) = 0 . and . , respectively) and stretch effect cannot explain thepeculiar red colors.We also investigate the relation between B − V color and Si II λ absorption line velocity.We use the Si II velocity measured within ± B − V and V − R colors have typical values of Si II velocity (SN 2000B has no velocitymeasurements). ∆ M B vs SN CE
In this section, we examine the ∆ M B and SN CE parameters introduced by TAK17. ∆ M B is sen-sitive to distance to an object, so nearby SN Ia has a large uncertainty of ∆ M B since it will beaffected by the peculiar velocity ( σ V ) of its host galaxy. Therefore, we plot samples whose totalmagnitude uncertainty, σ total = q σ m + σ phot. is less than 0.30 mag. σ m is a magnitude uncertaintycaused by the peculiar velocity σ m = σ V cz ln(10) , where c is the speed of light and z is the redshift. σ phot. is an uncertainty from photometry. Here we adopt a typical peculiar velocity in the local Universe σ V = 300 km / s and 23 SNe Ia in the Branch sample passed the criterion (hereafter referred to as"Branch sub-sample"). We plot the Branch sub-sample on the ∆ M B and SN CE diagram in figure 5.In TAK17, they showed statistically that the sample whose host color is blue (red) has a small(large) value of
SN CE and the both samples may obey multiple dust extinction laws. Using SNe Iawith typical stretch factor ( . < s ( B ) < . ), they obtained extinction law with R V = 2 . +0 . − . fromthe sample whose host color is red ( u − r > . ) and that with R V = 3 . +0 . − . from the sample whosehost color is blue ( u − r < . ). In figure 5, we show different dust extinction laws. Black solid linedenotes dust extinction similar to that of our Milky-Way ( R V = 3 . ) and red dashed line shows anextinction law with small R V = 2 . . Most of our samples become to be consistent with the Milky-Waylike extinction law after stretch correction. On the other hand, there are some outliers whose SN CE are large (SN 1999cl, 1999gh and 2000B) compared to the other objects with similar ∆ M B .In figure 6, we plot the ∆ M B and SN CE diagram colored by the host morphology. Theobjects whose hosts are early-type spirals (Sa-Sb) are distributed along with the Milky-Way like ( M B − M V ) = 0 . × s − B ) − . and ( M V − M R ) = 0 . × s − B ) − . B − V [mag] −0.20.00.20.40.60.81.0 V − R [ m a g ] SS (12)CN (9)BL (10)CL (3)R V = 3.1R V = 2.0 Fig. 1: B − V vs V − R color-color diagram. Different symbols show different Branch sub-types.Numbers in the brackets indicate the number of each Branch sub-type sample. Black solid and reddashed lines represent dust extinction laws with R V = 3 . (Milky-Way like) and R V = 2 . calculatedusing Cardelli et al. (1989), respectively. We draw these lines in order to pass across the main clusteraround B − V ∼ V − R ∼ .extinction line. Among the three BL outliers with peculiar red SN CE colors, two objects (SN 1999ghand 2000B) are occured in early-type (E/S0) galaxies. To increase sample size, we make a subset of67 objects from TAK08 with σ total < . mag (hereafter refereed to as "TAK08 photometric sub-sample") which will be discussed in section 4.3. The list of the TAK08 photometric sub-sample isshown in table 3. We added TAK08 photometric sub-sample shown by white filled circles in figure 6and 7. 9 B − V [ ag] −0.20.00.20.40.60.81.0 V − R [ a g ] s (B) = 0.8 → 1.2 E/S0 (9)Sa-Sb (15)Sc-Irr (10)R V = 3.1R V = 2.0 s t r e t c h f a c t o r s ( B ) Fig. 2: B − V vs V − R color-color diagram with host galaxy morphology and s ( B ) . Circles, pentagonsand stars indicate objects whose hosts are elliptical or lenticular galaxies (E/S0; T ≤ ), early-typespirals (from Sa to Sb; ≤ T ≤ ), and late-type spirals or irregular galaxies (Sc or later and Irr; T ≥ ), respectively. The color bar indicates the B − band stretch factor s ( B ) of SNe Ia. For reference,the blue arrow shows the variation of the bluest SNe Ia colors when s ( B ) varies from 0.8 to 1.2. Thelines presented here are the same as in figure 1. SN CE
In TAK17, they suggest that there may be different populations on the ∆ M B − SN CE diagram. Thatmeans there may be sub-types with intrinsically different color and dust extinctions. We examinedthe distribution of
SN CE difference between CN and BL sub-types, both of which have typicalvalues of stretch factor s ( B ) . We apply Kolmogorov–Smirnov (KS) test to check if their probabilitydistributions are different or not. As a result, we found the p-value is 0.41 and there is no difference intheir distributions at the 5% significant level. However, our sample is not enough in numbers, so it is10 a) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 B − V [mag] −0.20.00.20.40.60.81.0 V − R [ m a g ] SS wi h hos ype
E/S0 (2)Sa-Sb (5)Sc-Irr (5)R V =3.1R V =2.0 (b) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 B − V [mag] −0.20.00.20.40.60.81.0 V − R [ m a g ] CN with ho t type
E/S0 (3)Sa-Sb (3)Sc-Irr (3)R V =3.1R V =2.0 (c) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 B − V [mag] −0.20.00.20.40.60.8 V − R [ m a g ] BL with ho t type
E/S0 (3)Sa-Sb (6)Sc-Irr (1)R V =3.1R V =2.0 (d) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 B − V [mag] −0.20.00.20.40.60.8 V − R [ m a g ] CL with ho t type
E/S0 (1)Sa-Sb (1)Sc-Irr (1)R V =3.1R V =2.0 Fig. 3: B − V vs V − R diagrams with host galaxy morphology. Each figure shows the Branch sampleof (a) Shallow Silicon, (b) Core Normal, (c) Broad Line and (d) Cool objects, respectively. The redand black lines presented here are the same as in figure 1.desired to obtain the larger number of sample with Branch sub-type classification to test the validity.11
10 11 12 13 14 v Si II λ6355 [1000 km/ ] −0.20.00.20.40.60.81.01.2 B − V [ m a g ] SS (11)CN (9)BL (9)CL (2)
Fig. 4: The relation of the Si II λ absorption line velocity (hereafter v ) and B − V color. Thevalue of v is derived from Blondin et al. (2012). SN 1997br, 2000B and 2000cn have no velocitymeasurements. 12ig. 5: The ∆ M B − SN CE diagram of the Branch sub-sample. Black solid and red dashed linesdenote different dust extinctions with R V = 3 . and R V = 2 . respectively. The solid blue line showsthe average of linmix MCMC fit results to the 20 samples excluding three BL outliers (SN 1999cl,1999gh, and 2000B), with 100 random MCMC fit lines overlaid as light blue.13 ΔM B Δ[ma ] −1.0−0.50.00.51.01.52.0 S N C E V = 3.1R V = 2.0 Fig. 6: The ∆ M B − SN CE diagram colored by the host galaxy morphology. TAK08 photometricsub-sample is shown by white filled circles. We also include SN 2006X, a BL sub-type SN Ia withhigh-extinction for comparison (see section 4.2). 14ig. 7: The ∆ M B − SN CE diagram including the TAK08 photometric sub-sample marked as whitefilled circles. The green lines show the result of MCMC fit to all Branch sub-sample + TAK08photometric sub-sample while the blue lines show the result without the three BL outliers. For moredetailed descriptions of other lines, see the caption in figure 5. We also include SN 2006X, a BLsub-type SN Ia with high-extinction for comparison (see section 4.2).15 able 1: Photometric and spectroscopic information of the Branch sample and the host galaxies
SNname Branch ∗ sub-type s ( B ) † B ‡ V § R k EW(6100) (Å) EW(5750) ∗∗ (Å) v †† (km/s) Phase ‡‡ of spectrum (day) Wang §§ class Host kk name cz (km/s) Host ∗ ∗ ∗ morphology Host T index1989B CL 0.902(0.021) -18.69(0.04) -18.98(0.02) -19.20(0.02) 126 25 -10634 -1.3 N NGC 3627 727 SAB(s)b 31991T SS 1.119(0.025) -20.77(0.04) -20.92(0.02) -20.93(0.03) 27 3 -9660 -1.5 91T NGC 4527 1736 SAB(s)bc 41994D CN 0.838(0.010) -18.49(0.02) -18.38(0.01) -18.41(0.01) 98 21 -11248 -0.1 N NGC 4526 617 SAB(s)0^0 -21994ae CN 1.067(0.015) -18.68(0.01) -18.69(0.02) -18.79(0.01) 87 10 -10979 0.0 N NGC 3370 1279 SA(s)c 51996X CN 0.900(0.006) -19.63(0.01) -19.57(0.01) -19.56(0.01) 85 20 -11161 0.1 N NGC 5061 2082 E0 -51997br SS 0.845(0.016) -19.37(0.03) -19.31(0.01) -19.54(0.01) - - - - - ESO 576-40 2085 Sd/Irr 71997do BL 1.002(0.011) -18.95(0.02) -18.89(0.01) -18.97(0.01) - - -14332 -6.6 HV UGC 3845 3034 Sbc 41997dt CN 0.971(0.009) -16.66(0.02) -17.07(0.02) -17.41(0.01) 87 14 -11310 0.2 N NGC 7448 2194 Sbc 41998V CN 1.012(0.007) -19.31(0.01) -19.20(0.00) -19.25(0.01) 84 19 -10843 0.3 N NGC 6627 5268 Sb 31998ab SS 0.958(0.011) -19.35(0.02) -19.24(0.01) -19.36(0.01) - - -9209 6.7 91T NGC 4704 8134 Sc 51998bu CN 1.003(0.005) -19.09(0.01) -19.36(0.00) -19.54(0.01) 93 21 -11009 -1.6 N NGC 3368 897 Sab 21998dh BL 0.937(0.005) -18.71(0.01) -18.74(0.01) -18.82(0.01) 120 24 -12091 -0.6 N NGC 7541 2678 Sbc 41998ec BL 1.029(0.023) -18.52(0.04) -18.67(0.05) -18.66(0.02) 124 11 -12751 -3.0 HV UGC 3576 5966 Sb 31998eg CN 0.970(0.031) -18.93(0.02) -18.87(0.01) -18.94(0.01) 95 20 -10860 -0.8 N UGC 12133 7423 Sc 51998es SS 1.132(0.016) -19.21(0.01) -19.26(0.01) -19.28(0.01) 51 7 -10183 -0.5 91T NGC 632 3168 S0 01999aa SS 1.143(0.026) -19.33(0.02) -19.22(0.03) -19.20(0.01) 63 14 -10430 -0.2 91T NGC 2595 4330 Sc 51999ac SS 1.015(0.010) -19.01(0.01) -18.98(0.00) -19.03(0.00) 84 9 -9993 0.4 91T NGC 6063 2848 Scd 61999cc BL 0.850(0.012) -18.88(0.01) -18.80(0.01) -18.90(0.01) 121 26 -12047 -0.2 N NGC 6038 9392 Sc 51999cl BL 0.972(0.019) -17.92(0.02) -19.03(0.01) -19.57(0.02) 133 19 -12395 0.7 HV NGC 4501 2281 Sb 31999dq SS 1.094(0.004) -19.43(0.00) -19.45(0.00) -19.49(0.01) 51 10 -10680 0.4 91T NGC 976 4295 Sc 51999ee SS 1.124(0.016) -18.37(0.02) -18.60(0.08) -18.71(0.03) 77 9 -10018 -0.2 N IC 5179 3422 SA(rs)bc 41999ej BL 0.792(0.021) -18.33(0.03) -18.26(0.01) -18.32(0.02) 108 27 -10598 1.1 N NGC 495 4114 S0/Sa 01999gd BL 0.942(0.010) -17.64(0.02) -17.97(0.01) -18.23(0.02) 108 17 -10713 1.1 N NGC 2623 5535 Sa 11999gh BL 0.756(0.016) -18.62(0.02) -19.12(0.03) -19.07(0.01) - - -11467 5.5 N NGC 2986 2302 E -51999gp SS 1.204(0.008) -19.22(0.01) -19.19(0.00) -19.21(0.01) 52 8 -11332 0.0 91T UGC 1993 8018 Sb 32000B BL 0.854(0.029) -18.90(0.03) -19.14(0.06) -19.00(0.01) - - - - - NGC 2320 5901 E -52000E SS 1.101(0.003) -18.52(0.00) -18.62(0.00) -18.66(0.00) 78 13 -10979 -2.3 N NGC 6951 1424 SAB(rs)bc 42000cn CL 0.761(0.009) -18.44(0.01) -18.53(0.01) -18.62(0.01) - - - - - UGC 11064 7043 Scd 62000cx SS 0.863(0.055) -19.34(0.06) -19.41(0.10) -19.29(0.06) 49 3 -11844 -0.3 * NGC 524 2379 S0 02000dk CL 0.762(0.008) -18.87(0.01) -18.83(0.01) -18.90(0.01) 124 47 -11035 0.8 N NGC 382 5228 E -52001V SS 1.174(0.021) -19.56(0.02) -19.58(0.03) -19.57(0.02) - - -11449 -3.3 91T NGC 3987 4361 Sb 32001el CN 0.953(0.012) -18.14(0.01) -18.20(0.01) -18.38(0.01) 95 14 -11544 1.2 N NGC 1448 1164 SAcd 62002bo BL 0.951(0.024) -17.92(0.02) -18.25(0.01) -18.51(0.04) 147 13 -13198 -0.6 HV NGC 3190 1271 SA(s)a pec 12004S CN 0.946(0.016) -18.93(0.02) -18.78(0.03) -18.88(0.02) 89 13 -9256 1.6 N MCG -05-16-21 2516 S -22006X ††† BL 0.858(0.025) -15.51(0.14) -16.87(0.14) -17.41(0.14) 179 9 -15680 1.3 HV NGC 4321 1557 Sbc 4 ∗ Branch et al. (2009). † , ‡ , § , k Takanashi et al. (2008). The 1 σ error is given in between parenthesis. , ∗∗ Branch et al. (2009). †† , ‡‡ , §§ Blondin et al. (2012). †† It is the phase of v measurement relative to B − band maximum. kk Branch et al. (2009) and Transient Name Server
First, we discuss the B − V , V − R color-color diagram. In figure 2, the distributions with hostmorphology show that SNe Ia that occur in early-type spirals, rather than late-type spirals, tend tohave redder colors along with the dust extinction lines. The distribution trend mentioned above holdswhen divided into Branch sub-types (see figure 3). In figure 1 of Childress et al. (2013), using 115SNe Ia from the Nearby Supernova Factory, they compared SNe Ia color, host galaxy stellar mass,specific star formation rate (sSFR) and metallicity. They found that SNe Ia with red color belong togalaxies with intermediate stellar mass and sSFR. Our findings are consistent with their result. In theBranch CN sample, there is no outlier object which deviates from the extinction lines. On the otherhand, there are three outliers in BL sample (SN 1999cl, 1999gh and 2000B) and the two of them(SN 1999gh and 2000B) occur in elliptical galaxies. This implies that we can roughly distinguishCN and peculiar red BL SNe Ia with a color-color diagram. Mandel et al. (2014) reported that BLhigh-velocity (=HV) SNe Ia show intrinsic redder B − V color (by ∼ . mag) than normal-velocityevents. In addition, smaller R V are predominantly derived from HV SNe Ia (Wang et al. 2009) andsome of our BLs in figure 1 are consistent with their results. ∆ M B − SN CE diagrams
We discuss some outliers on the color-color and ∆ M B − SN CE diagrams. First we focus on theBL sub-type outliers: SN 1999cl, 1999gh and 2000B. The Si II λ velocity of SN 1999gh is v = − , km/s, which is relatively small among BL sub-type (SN 2000B has no measurements).The boundary between NV and HV SNe Ia is located at ∼ − , km/s (see figure 8 of Blondinet al. 2012). There is a large overlap between the CN and the NV SNe Ia, as well as between theBL and HV SNe Ia. So the Si II velocity may not be a major quantity to discriminate peculiar BLobjects. SN 1999gh and 2000B have red color in B − V but bluer in V − R than SS objects. The hostmorphological types of the SN 1999gh and 2000B are both elliptical galaxies ( T = 5 ), so they arein generally less-dusty environment. Therefore, it can be inferred that SN 1999gh and 2000B haveintrinsic red B − V colors considering their blue V − R colors. Mandel et al. (2014) shows similarresult that intrinsic color distributions of HV and normal SNe Ia exhibit significant discrepancies in B − V and B − R colors.SN 1999cl has distinct red color ( B − V = 1 . , V − R = 0 . ) compared with the other BLobjects. Its host galaxy type is Sb ( T = 3 ), which often shows strong extinction. From the imageof its discovery, SN 1999cl is located in the disk component of its host. The Si II λ velocity20f SN 1999cl is v = -12,395 km/s, which is typical among BL sub-type, but the overall shapeof its spectrum is much redder than others. Based on these facts, SN 1999cl can be suffered fromsignificant host extinction. In Krisciunas et al. (2006), they obtained the value of R V = 1 . for SN1999cl, which may imply the size of dust is very small ( ∼ − µm ). But when we compare SN1999cl with SN 1999gh and 2000B on the ∆ M B − SN CE diagram, another possibility is that SN1999cl has intrinsically red color, and also is strongly reddened by Milky-Way type dust extinction.Given the distribution trend of sample with Sa-Sb host galaxies seen in figure 6, the latter is morelikely. We compared SN 1999cl with SN 2006X, another high-extinction SN Ia (Wang et al. 2008)and also is classified as BL (EW(6100), EW(5750) = 179, 9) in Branch et al. (2009). The host of SN2006X (= NGC4321) is an Sbc-type galaxy, which is similar to that of SN 1999cl. In Blondin et al.(2009), they report that the variability in Na I D lines was found both in the spectra of SN 1999cl and2006X. The Na I D variability is interpreted to be associated with CSM in the SN Ia progenitor system(Patat et al. 2007). SN 2006X has B − V = 1 . and V − R = 0 . colors at B − band maximum (Wanget al. 2008). The V − R color of SN 2006X is nearly the same as SN 1999cl but the B − V color is ∼ . mag redder than that of SN 1999cl. The ∆ M B and SN CE of SN 2006X are ∆ M B = 3 . ± . and SN CE = 1 . ± . (see figure 6 and 7). The slope between SN 1999cl and 2006X is closeto the slope of R V = 3 . line on the ∆ M B − SN CE diagram, suggesting that both SN 1999cl and2006X have intrinsically red color (possibly by CSM) and they are highly extincted by interstellardust. While most SS objects show bluer color in both B − V and V − R , SN 1997br has bluer colorin B − V but redder color in V − R (the fourth reddest color in the Branch sample in figure 1). Its hostgalaxy type is Sd/Irr ( T = 7 ) and the morphological index is the largest in our sample, suggesting thatthe SN 1997br is in dusty environment. However, if considering its bluer color in B − V , the peculiarcolor may be intrinsic. In Li et al. (1999), they reported that the light curve and time evolution ofthe spectra of SN 1997br are similar to those of over-luminous type of SN 1991T. However, theirlocations in color-color diagram are different.SN 1997dt suffers highest extinction but has a typical value of stretch factor ( s ( B ) = 0 . )among CN sample. In addition, the spectral features of SN 1997dt look similar to the other CNs butthe flux of the spectrum was reduced in shorter wavelengths. Its host galaxy type is an early-spiralSbc ( T = 4 ) which often shows strong extinction. We used the Cepheid-based distance modulus of . ± . mag (Freedman et al. 2001) for calculating ∆ M B . .3 Implications for intrinsic color of SNe Ia Since ∆ M B and SN CE are residuals from the stretch-magnitude/color relations of the bluest SNe Ia, ∆ M B − SN CE diagram tells us the information of intrinsic luminosity/color and extinction by dust.It is clear that there are three BL objects (SN 1999cl, 1999gh and 2000B) which have large
SN CE infigure 5. This means they show red color after correcting for s ( B ) . As shown in figure 2, these threeBLs have small stretch factors in the Branch sample. When stretch correction is performed, theircolors move in the lower-left direction in figure 2 (shown as the blue arrow). In addition, two-thirdsof the BLs (SN 1999gh and 2000B) occur in early-type (E/S0) galaxies, suggesting that BL objectswhose hosts are E/S0 galaxies need to apply different stretch correction. Another possibility is thatthe progenitor scenarios (explosion mechanisms) of such BL objects may differ from typical SNe Ia.It is possible that small R V can be derived because the R V is averaged both for normal SNeIa and intrinsically red SNe Ia. We employ the Bayesian linear regression method by Kelly (2007)with its python package linmix . In figure 5, the solid blue line shows the average of linmix MCMCfit results to the Branch sub-sample excluding three BL outliers (SN 1999cl,1999gh and 2000B) with100 random MCMC fit lines overlaid as light blue. As a result, the average regression line whichcorresponds to R V = 3 . +0 . − . well matches the Milky-Way like extinction with R V = 3 . .To make the result more robust, we include the TAK08 photometric sub-sample for the re-gression analysis. In figure 7, the green solid line shows the average of 100 MCMC fit results usingall Branch sub-sample and TAK08 photometric sub-sample while the blue line shows the same resultwithout three BL outliers (SN 1999cl, 1999gh and 2000B). The slope of the green line correspondsto R V = 2 . ± . , while that of the blue line is R V = 3 . +0 . − . which is almost identical to R V = 3 . .Therefore the three BL outliers make R V small. When we exclude such outliers, the extinction lawfor SNe Ia becomes close to R V = 3 . .When we use SNe Ia for cosmological studies to measure distance, excluding peculiar BLobjects may give more accurate results. This could be done just to use two optical colors (e.g., B − V and V − R ) around the time of maximum brightness. In addition, further environmental studies willgive us clues about the difference in progenitor scenarios among Branch sub-types.Using early phase color information, Stritzinger et al. (2018) found that there are two distinctearly populations with different early color evolution in B − V and all blue events are of the Branch SSsub-type, while all early red events except for the peculiar 2000cx-like SN 2012fr are of the BranchCN or CL sub-types. It is also important to study the early color evolution with Branch sub-types. https://github.com/jmeyers314/linmix Summary
In this study, with Branch spectroscopic classification as well as host galaxy morphology, we inves-tigate the diversity of color and dust extinction of nearby 34 SNe Ia ( z < ∼ . ). We summarize ourfindings as below.• In the B − V, V − R color-color diagram, different distribution among different host galaxy mor-phology can be seen: SNe Ia which occur in early-type spirals have the reddest color. The distribu-tion trend holds when divided into the Branch spectroscopic sub-types.• The three BLs (SN 1999cl, 1999gh and 2000B) have red colors in the ∆ M B − SN CE diagram.Two of them (SN 1999gh and 2000B) can be explained by their intrinsic red colors, and their hostsare elliptical galaxies which usually have little interstellar dusts. On the other hand, 1999cl whichoccur in Sb type host can be explained by both its intrinsic red color and strong extinction byinterstellar dust.• In our sample, it is inferred that lower value of R V parameter is not necessary to explain the colordiversity of SNe Ia. In other words, it suggests that the extinction law for most of SNe Ia might beexplained by the typical extinction law in the Milky Way ( R V = 3 . ).Our results suggest the possibility of typical host galaxy extinction law for SNe Ia as seen in theMilky Way. It was suggested by TAK17 that there seems to be two (or more) different sub-groupswith different intrinsic colors and/or dust extinction laws, but we infer this may be caused by someobjects with peculiar intrinsic red color. We thank the anonymous referee for reading the paper carefully and providing thoughtful comments,many of which have resulted in changes to improve the revised version of the manuscript. Thiswork was supported by JSPS KAKENHI Grant numbers 18H05223 and 18H04342. N.A. gratefullyappreciates the financial support of Hattori International Scholarship Foundation (HISF) for a grantthat made it possible to complete this study.
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