Formation of an optically thick shocked shell in the very fast nova V1674 Herculis: the origin of superbrightness

TL;DR

V1674 Her is the fastest ($t_2 \sim 1$ day) and brightest ($M_{V,\rm max} \sim -10.2$) classical nova observed in the Galaxy, classified as a superbright nova. The authors develop a fully self-consistent nova outburst model that includes optically thick winds on a $1.35~M_\odot$ WD and, crucially, an optically thick shocked shell that drives the optical peak, while GeV gamma-rays originate from the shock. They show that the gamma-ray maximum at $t \sim 0.4$ d precedes the optical maximum at $t \sim 0.7$--$0.8$ d because the shocked-shell photosphere, not the inner free-free wind emission, sets the peak brightness. The analysis is extended to V1500 Cyg using a time-stretching method, revealing a shared mechanism for superbright novae and yielding a consistent distance to V1674 Her of about $d \approx 8.9$ kpc. Together, the results provide a coherent, multiwavelength explanation for the unusually bright peak and the observed chronology of high-energy and optical emissions in very fast novae.

Abstract

V1674 Her is the fastest ($t_2\sim 1$ day) classical nova in our Galaxy and its absolute $V$ peak of $M_{V,\rm max}\sim -10.2$ is one magnitude brighter than typical very fast novae. Such a nova is sometimes called a superbright nova. Using our fully self-consistent nova outburst model combined with the optically thick winds on a $1.35 ~M_\odot$ white dwarf (WD) with a mass accretion rate of $1\times 10^{-11} ~M_\odot$ yr$^{-1}$, we have clarified that a strong reverse shock arises $0.3$ days after the outburst, which is just after the maximum expansion of the WD photosphere. The shocked shell is optically thick and expanding with the velocity of $\sim 3500$ km~s$^{-1}$. Its $V$ brightness reaches maximum of $M_{V,\rm max}=-10.2$ when the shocked shell expands to $R_{\rm shell}\sim 300 ~R_\odot$ on day $\sim 0.7$. After that, the shocked shell turns to optically thin and becomes fainter than the brightness of free-free emission from the nova wind. In chronological order, the optical brightness of free-free emission reaches maximum of $M_V=-9$ on day 0.3. However, it is overtaken on day 0.5--0.7 by the $\sim$1 mag brighter luminosity of the optically thick shocked shell. The GeV gamma-ray flux reaches maximum on day 0.4 because the gamma-rays are emitted by the shock that arises on day 0.3. Our model consistently explains both the superbrightness and chronological order that the gamma-ray peak precedes substantially before the optical $V$ peak. We also present a similar light curve model for another superbright nova V1500 Cyg.

Figures (10)

• Figure 1: Summary of the visual, $V$, $g$, and X-ray (0.3--10.0 keV) light curves of V1674 Her for both models and observations. The discovery date is indicated by the downward black arrow labeled "discovery." The $V$ and visual data are taken from the archive of the American Association of Variable Star Observers (AAVSO). The All-Sky Automated Survey for Supernovae (ASAS-SN) $g$, Evryscope $g$, and Itagaki's unfiltered CCD data are from qui24. The X-ray count rates are from the Swift website eva09. We add theoretical $V$ (black line) and X-ray (magenta line) light curves based on kat25hs's fully self-consistent nova outburst model. We set our theoretical outburst day ($t=0$ at epoch B in their Figure 1(a)) to be HJD 2,459,377.68($=$UT 2021 June 12.18). The WD model has the mass of $M_{\rm WD}= 1.35 ~M_\odot$ with the mass-accretion rate of $\dot{M}_{\rm acc}= 1\times 10^{-11} ~M_\odot$ yr$^{-1}$. The model $V$ light curve (black line) is calculated from free-free emission from nova winds hac25kv1674her2 whereas the model X-ray light curve (magenta line) is calculated from the blackbody emission from the WD photosphere (0.3--10.0 keV). The thick orange line shows the photospheric $V$ light curve of the WD, accretion disk, and companion star and the light gray line corresponds only to the WD photosphere, which are taken from hac25kv1674her2. The straight thick cyan line labeled $t^{-1.75}$ denotes the universal decline law of $L_V\propto t^{-1.75}$hac06kb, where $L_V$ is the $V$ band luminosity. The $V$ band distance modulus $\mu_V\equiv (m-M)_V= 16.3$, the distance $d=8.9$ kpc, and the extinction $E(B-V)=0.5$ toward V1674 Her are taken from kat25hs. There is a gap between the theoretical free-free emission model light curve (black line) and the observation, as demonstrated in the yellow-shadowed area. We also show the pre-outburst brightness of $g = 19.17$qui24 1.7 days before the nova outburst ($t=-1.7$ day). See the main text for more detail.
• Figure 2: Maximum $V$ magnitude versus rate of decline (MMRD) diagram, $\log (t_2)$-$M_{V,\rm max}$, for classical novae. (a) The blue lines indicate theoretical model equi-WD mass lines, from left to right, 1.35, 1.3, 1.25, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, and $0.6~M_\odot$; the thick solid gray lines denote model equi-mass accretion rate ($\dot M_{\rm acc}$) lines, from lower to upper, $3\times 10^{-8}$, $1\times 10^{-8}$, $5\times 10^{-9}$, $3\times 10^{-9}$, $1\times 10^{-9}$, $1\times 10^{-10}$, and $1\times 10^{-11} M_\odot$ yr$^{-1}$; the red lines represent model equi-recurrence time lines, from lower to upper, $t_{\rm rec}= 30$, 100, 300, 1000, 10000, $10^5$, $10^6$, and $10^7$ yr. These lines are taken from hac20skhs based on the optically thick nova wind model kat94h and nuclear runaway model calculation of mass accretion onto each WD. The brightnesses of novae are calculated from free-free emission luminosity of Equation (\ref{['free-free_flux_v-band']}). The thick yellow line corresponds to the $x_0\equiv M_{\rm env}/ M_{\rm sc} =2$ line, where $M_{\rm env}$ is the hydrogen-rich envelope mass at the optical maximum and $M_{\rm sc}$ the scaling mass. Their assumed scaling law for $\dot{M}_{\rm wind}$ and $M_{\rm env}$ is valid only for $x_0\gtrsim 2$. Therefore, below the yellow line ($x_0<2$), the brightnesses of these models are not accurate hac20skhs. We overplot filled red circles taken from "Golden sample" of schaefer18, filled stars from sel19, and open star (V1500 Cyg) from del20i. The three novae (KT Eri, V339 Del, and V392 Per) are taken from hac25kw, hac24km, and hac25kv392per, respectively. The thick solid cyan line indicates the empirical line for the MMRD relation obtained by del20i. The two novae, V1674 Her (orange triangle) and V1500 Cyg (unfilled red star), are located outside the region of hac20skhs. The peak brightnesses of these two novae cannot be reproduced by the free-free emission model light curves, which indicates that the energy source is different from free-free emission. See Sections \ref{['evolution_shocked_shell']} and \ref{['light_curve_v1500_cyg']}, respectively, for their reasons. (b) Same as panel (a), but we show only the position of each nova and empirical MMRD line of del20i. The thick cyan line indicates the same as the thick cyan line in panel (a), and light-gray shadow line corresponds to its $\pm 0.5$ mag region. The blue line is 1 mag above the thick cyan line.
• Figure 3: Cartoon for our V1674 Her nova model in the early phase. (a) The nova (WD) photosphere expands over $R_{\rm ph} \sim 0.1 ~R_\odot$ and optically thick winds are accelerated deep inside the photosphere kat22shakat25hs. The wind itself becomes optically thin outside the photosphere. The nova (WD) photosphere is further expanding. The earliest wind forms the pre-maximum absorption/emission line system mcl42 outside the WD photosphere ($r > R_{\rm ph}$). (b) After maximum expansion of the nova (WD) photosphere, the photosphere is receding. A strong shock arises outside the WD photosphere hac22k. The shocked shell is so dense that the optical depth $\tau_{\rm shell}$ of the shell is larger than unity (optically thick) just after the shock arises. The shocked shell emits gamma-rays. (c) The shocked shell is further expanding and its optical depth $\tau_{\rm shell}$ gradually decreases to less than unity (optically thin). The shocked shell is geometrically thin and optically thin. The whole ejecta is divided into three parts, outermost expanding gas (earliest wind), shocked shell, and inner wind. These three parts contribute to pre-maximum, principal, and diffuse enhanced absorption/emission line systems mcl42, respectively, as proposed by hac22khac23k. The velocity of principal system is typically about a half of that of diffuse enhanced system mcl42hac22k. The optically thin shocked shell emits thermal hard X-rays. (d) An enlargement of the shocked layer in panel (b). We plot locations of the reverse shock, hydrogen recombination front (photosphere), outermost edge of the shocked shell (optically thin layer). The photosphere of the shocked shell emits photons like a supergiant.
• Figure 4: (a) The early $V$, visual, and $g$ light curve of V1674 Her (black symbols) and V339 Del (small and large filled red circles). The data on V339 Del are taken from Figure 2 of hac24km. The timescale of V1674 Her is expanded by 7.55 and its $V$ magnitude is shifted up by $\Delta V= 1.4$. The $V$, visual, and $g$ data of V1674 Her are the same as those in Figure \ref{['v1674_her_v_x_observation_only_logscale']} and correspond to the yellow-shaded region in Figure \ref{['v1674_her_v_x_observation_only_logscale']}. We add $B-V$ color evolution of V339 Del (open magenta diamonds). The large symbols of V339 Del denote the data taken from mun15mm, bur15a, SMARTS wal12bt, and OKU hac24km, while the small filled red circles are taken from AAVSO. The broad gray line indicates our approximations of the $V$ magnitude. The vertical broad cyan line denotes the epoch of the global optical peaks of V339 Del, i.e., Aug. 18.47$\pm$0.11 sko14dt. (b) The temporal developments of the effective temperature $T_{\rm eff}$, luminosity $L_{\rm WD}$, and radius $R_{\rm WD}$ of the pseudo-photosphere, taken from sko14dt. The broad gray lines indicate our approximations to the temporal developments of each value when we exclude the early fluctuations in the data of V339 Del. The luminosity and radius depend on the assumed distance to the nova. sko14dt assumed $d=3$ kpc, so that the luminosity and radius should be translated from the original values to the true values according to $L_{\rm WD} \propto (d/3{\rm ~kpc})^2$ and $R_{\rm WD} \propto (d/3{\rm ~kpc})$, respectively. The three thin blue, red, black lines connect each data. We also add GeV gamma-ray fluxes ack14aa. ack14aa assumed $d=4.2$ kpc, so the gamma-ray luminosity depends on $L_{\gamma} \propto (d/4.2{\rm ~kpc})^2$. hac24km determined the distance of V339 Del to be 2.1 kpc.
• Figure 5: (a) The evolution of absolute $V$ brightness, $M_V$, of the shocked shell near optical maximum in V1674 Her. The optical data are the same as those in Figure \ref{['v1674_her_v_x_observation_only_logscale']}, but we added the CV magnitudes (open blue triangles) taken from ayd21sc. The orange, thick red, and magenta lines represent different initial radii of the shocked shell models having the same temperature in panel (b), respectively. We also add the optical depth $\tau$ (green line) of the shocked shell for the shell mass of $M_{\rm shell}= 3\times 10^{-6} ~M_\odot$. (b) The assumed evolutions of the photospheric temperature (green line) and radii (orange, thick red, and magenta lines) for our shocked shell models of V1674 Her, mimicking the evolutions of V339 Del sko14dt as in Figure \ref{['v1674_her_v339_del_v_skopal_wd_photo']}(b). The green circle denotes the observed color temperature of $(B-V)_0=0.169$ on day 0.72 mun21vd. Here, we assume the same temperature (green line) evolution for three different initial radii models, each of which starts at $R_{\rm ph,sh}= 240 ~R_\odot$ (orange line), $220 ~R_\odot$ (thick red line), and $180 ~R_\odot$ (magenta line) and expands with the same velocity of $v_{\rm shell}= 2600$ km s$^{-1}$. The best fit light curve (thick red line) for $v_{\rm shell}=$2600 km s$^{-1}$ is represented numerically by $T_{\rm ph, sh} = 10000$ K for $t< 0.6$ days whereas $T_{\rm ph, sh} = 10000 - 8000(t-0.6)$ K for $t\ge 0.6$ days and $R_{\rm ph, sh}= 220 ~R_\odot + 2600{\rm ~km~s}^{-1} \times (t-0.3)$ days for $t\ge 0.3$ days. (c) Same as in panel (a), but for the expansion velocity of $v_{\rm shell}= 4000$ km s$^{-1}$. The three $M_V$ lines (orange, thick red, and magenta lines) correspond to the initial radii of the same color in panel (d). (d) Same as in panel (b), but for the expansion velocity of $v_{\rm shell}= 4000$ km s$^{-1}$ with the initial radii of $R_{\rm ph,sh}= 240 ~R_\odot$ (orange), $210 ~R_\odot$ (thick red), and $180 ~R_\odot$ (magenta). Here, the best fit light curve (thick red line) for $v_{\rm shell}=$4000 km s$^{-1}$ is represented numerically by $T_{\rm ph, sh} = 10000$ K for $t< 0.5$ days whereas $T_{\rm ph, sh} = 10000 - 8000(t-0.5)$ K for $t\ge 0.5$ days and $R_{\rm ph, sh}= 210 ~R_\odot + 4000{\rm ~km~s}^{-1} \times (t-0.3)$ days for $t\ge 0.3$ days.