NOVA Labels: A Multidimensional Nova Taxonomy

• NOVA Labels are systematic, multifaceted classification schemes that quantitatively categorize novae based on photometric, spectral, morphological, and progenitor diagnostics.
• They employ high-cadence photometry, spectroscopy, imaging, and isotopic assays with explicit criteria—such as light-curve morphology and decline times—to achieve precise, reproducible labels.
• By unifying diverse observational and model-derived parameters, NOVA Labels facilitate robust cross-survey analyses, population synthesis, and evolutionary inferences in nova astrophysics.

NOVA Labels are systematic, multifaceted classification schemes used to assign taxonomic, physical, morphological, and paternity designations to novae and their related phenomena based on empirical measurements and model-derived criteria. They encompass a variety of dimensions—including photometric decline rate, light-curve morphology, spectral class, remnant structure, system configuration, and nucleosynthetic fingerprinting—each with explicit quantitative boundaries and decision rules sourced from high-cadence photometry, spectroscopy, imaging, or isotopic assays. The application of these labels ensures precise cross-comparison both within and across nova subpopulations and is foundational for population synthesis, astrophysical modeling, and evolutionary inference.

1. Photometric Morphological Labeling

A fundamental NOVA labeling axis is the quantitative classification of light-curve morphology, particularly as established by the Strope et al. (2010) scheme (Strope et al., 2010), widely adopted and extended in large surveys (e.g., WeCAPP (Lee et al., 2011)). This taxonomy encodes both temporal evolution (via $t_3$, the time to decline by 3 magnitudes) and explicit morphotypic structures.

Principal photometric classes and their features:

Class (Code)	Defining Features	Example(s)
S (Smooth)	Monotonic, broken power-law decline; no deviations >0.1 mag above photometric noise	V1500 Cyg = S(4)
P (Plateau)	Marked flattening (“plateau”) at mid-decline before a steep terminal drop	U Sco = P(3)
D (Dust Dip)	Sharp deep minimum (often >1 mag) followed by recovery; arises from dust condensation	DQ Her = D(100)
C (Cusp)	Distinct secondary maximum (“cusp”) typically 1–8 months post-primary; followed by steep decline	V2362 Cyg = C(246)
O (Oscillation)	Superposed quasi-periodic fluctuations, 0.5–1.5 mag, emerging $\sim$3 mag below peak	GK Per = O(13)
F (Flat Top)	Prolonged phase of near-constant maximum light, $<$0.1 mag variation over tens–hundreds of days	BT Mon = F(182)
J (Jitter)	Random, sharp-topped flares >0.5 mag, confined to early decline phase	HR Del = J(231)

Label usage is of the form <Class>(t3), e.g., P(10), corresponding to plateau morphology with $t_3=10$ d.

Speed class thresholds (Payne‐Gaposchkin/Warner):

• Very fast: $t_2<10$ d, $t_3<20$ d
• Fast: $10< t_2 <25$ d, $20< t_3 <40$ d
• Moderately fast: $25< t_2 <80$ d, $40<t_3<150$ d
• Slow/very slow: $\sim$0 d, $\sim$1 d

The "universal" decline law and related empirical relations (e.g., $\sim$2) are used to calibrate absolute magnitude at peak and assign luminosity-based labels (Chochol et al., 2020). For instance, CN Per 2018 with $\sim$3 d, $\sim$4 d is a “very fast” nova.

2. Spectroscopic, Structural, and Physical Subclassifiers

Spectral type labels (as per Williams 1992 (Chochol et al., 2020)):

Class	Criteria	Example
Fe II	FWHM lines $\sim$5 km/s, low ionization, strong Fe II, P-Cyg profiles	Typical in slow novae
He/N	FWHM $\sim$6 km/s, high He I/N II abundances, high ionization, boxy profiles	CN Per 2018
Hybrid	Evolution from Fe II $\sim$7 He/N (or vice versa)	Some fast novae

Geometric/kinematic structure:

Ejecta geometry is labeled via equatorial ring/bipolar flow models, often constrained by line profile decomposition, SHAPE modeling, and radio mapping. Inclination is recovered via $\sim$8; e.g., CN Per 2018 with $\sim$9 and emission-line profile modeling indicating denser receding polar flows (Chochol et al., 2020).

WD composition and mass labeling:

Inferred via nebular [Ne V] lines and calibrated empirical scaling (e.g., $<$0); e.g., ONe WD with $<$1 for CN Per 2018.

3. Remnant Morphology and Shell Evolution

Recent imaging catalogues provide a comprehensive morphological classification for nova remnants (Santamaría et al. 2025 (Santamaria et al., 25 Mar 2025)). The primary (upper-case) and secondary (lower-case) codes signify shell geometry and finer substructure, respectively:

Primary	Shape Criteria
R	Round, $<$2
E	Elliptical, $<$3
B	Bipolar, hourglass-like with polar lobes
A	Asymmetric, global tilt or brightness offset
I	Irregular, disordered/fragmented
S	Stellar, unresolved; no shell detected
G	Barely resolved (“Gaussian” excess over PSF)

Secondary	Features
s	Smooth surface brightness
c	Clumpy, pronounced knot structure
f	Filamentary features
t	Tails, radial structures
m	Multiple shells (concentric)
e	Equatorial brightness enhancement
x	Mixed (no dominant substructure)

The aspect ratio distribution follows a near-unity Gaussian with $<$4; the radius–age relation $<$5 (pc, yr) up to $<$6 yr provides a compact evolutionary epoch/physical scale label. Example: “Ect” for a clumpy, tailed elliptical shell.

4. System Configuration and Quiescent-Nova Labels

A parallel labeling axis derives from progenitor system demographics, categorizing novae by secondary evolutionary state (Darnley et al., 2013):

• MS-nova: Main-sequence donor ($<$7, $<$8)
• SG-nova: Subgiant donor ($<$9, $t_3=10$0)
• RG-nova: Red giant or symbiotic donor ($t_3=10$1, $t_3=10$2)

Discrimination employs quiescent near-IR color-magnitude position, orbital period, and spectral diagnostics. The system type label is independent of outburst light-curve but highly predictive of recurrence rate, outburst properties, and SN Ia paternity potential.

5. Nucleosynthetic Paternity and NOVA Labels for Presolar Grains

NOVA labeling is extended to presolar stardust grains by matching measured isotopic signatures to Monte Carlo nova nucleosynthesis simulations (Iliadis et al., 2018). The labeling workflow:

1. Measure presolar grain isotopic ratios (C, N, O, Ne, Mg, Si, S).
2. Sample nova model parameters—including WD composition, pre/post-explosion mixing fractions, peak $t_3=10$3/$t_3=10$4, and explosion timescales—via a one-zone nucleosynthesis network.
3. Define a match when simulated isotopic ratios overlap expanded intervals factoring in theoretical and measurement uncertainties: $t_3=10$5, $t_3=10$6 empirical broadening.
4. Labeling criteria:
   • $t_3=10$710% matching runs, $t_3=10$84 ratios: “High-probability CO nova”
   • 0.01%–10% matching, $t_3=10$92 ratios: “Possible CO nova”
   • Only matches with $t_2<10$0: “Low-probability/potentially diluted nova”
   • No matches (even for $t_2<10$1): “Exclude CO nova origin”
5. Report best-fit parameters ($t_2<10$2, $t_2<10$3, $t_2<10$4, $t_2<10$5) for astrophysical context.

This systematic, probabilistic labeling schema allows robust identification or exclusion of nova paternity for individual grains.

6. Specialized Subclasses: V1500 Cyg Stars and Recurrent Nova

V1500 Cyg stars constitute a physically distinct class labeled by a suite of interconnected observational thresholds (Schaefer et al., 2010):

• Eruption brightening $t_2<10$6 mag (post-eruption $t_2<10$7 pre-eruption flux)
• Short $t_2<10$8 hr
• Long-lived supersoft X-ray emission $t_2<10$9 yr
• High WD magnetic field ($t_3<20$0–100 MG)
• Secular post-eruption brightness decline $t_3<20$1–0.1 mag yr$t_3<20$2

Only $t_3<20$314% of novae are V1500 Cyg-type, and this subclass is not a SN Ia progenitor channel.

Recurrent nova (RN) labels are assigned based on positional coincidence and identified outburst history, sometimes supplemented by system configuration (MS/SG/RG-nova) and LC morphology (frequent: P-type with plateaus).

7. Summary Table: Multifaceted NOVA Labeling

Label Domain	Example Value / Assignment	Quantitative Basis or Criteria
Photometric speed	t$t_3<20$4=3 d, t$t_3<20$5=10 d ⇒ “fast” (very fast)	Decline times, Strope et al.
Light curve type	P(10): plateau at t$t_3<20$6=10 d	Morphologic analysis
Spectral class	He/N (FWHM=5600 km/s)	Line widths, ionization
Remnant morphology	Ect: elliptical, clumpy, tails	Imaging-based assignment
System config.	SG-nova: subgiant donor	Quiescent CMD, period
Paternity (grains)	“High-prob. CO nova”	Monte Carlo isotopic fit
V1500 Cyg star	All five properties above met	See section 6

NOVA Labels thus provide a multidimensional, quantitative taxonomy for the empirical and synthetic study of nova explosions and their progenitors, synthesizing information from time-domain photometry, spectroscopy, imaging, and nucleosynthetic modeling. These labels facilitate robust cross-survey analyses and underpin a unified approach in nova astrophysics (Strope et al., 2010, Chochol et al., 2020, Iliadis et al., 2018, Lee et al., 2011, Darnley et al., 2013, Santamaria et al., 25 Mar 2025, Schaefer et al., 2010).