Type Ib SupernovaEdit

Type Ib supernovae are a class of stellar explosions that mark the final, violent fate of certain massive stars after they have shed most or all of their hydrogen envelopes. They belong to the broader family of core-collapse supernovae and to the subset known as stripped-envelope supernovae, which also includes Type Ic supernovae. The defining observational feature of Type Ib events is the absence of hydrogen lines in their optical spectra, combined with clear helium lines, indicating helium-rich material in the outer layers of the exploding star. This combination points to progenitors that have lost hydrogen but retain helium, typically through strong mass loss either in single massive stars or via interaction with a binary companion.

Classification and defining features

Spectral signature: Type Ib supernovae lack hydrogen absorption features and show prominent helium lines (notably He I 5876 Å and related lines) in their early spectra.
Comparison with related classes: Type II supernovae retain hydrogen lines; Type Ic supernovae lack both hydrogen and helium lines, while Type IIb show hydrogen early on but transition to helium-dominated spectra. These related classes are collectively referred to as stripped-envelope supernovae, which also include Type Ib/c events when helium is weak or undetectable.
Light curves and energy source: The light emitted by Type Ib events is powered by the decay of nickel-56 to cobalt-56 and then to iron-56, similar to other core-collapse supernovae. The rise to maximum light and the subsequent decline reflect the amount of radioactive nickel synthesized in the explosion and the opacity of the ejected material.
Spectral evolution: As the ejecta expand and thin, the spectra transition from photospheric to nebular phases, revealing deeper layers. He I lines can persist into the photospheric phase, while the strength and presence of other elements (such as calcium and oxygen) provide clues to the composition and kinematics of the ejecta.

Progenitors and evolution

Single-star channel: A leading scenario involves very massive stars that lose their hydrogen envelopes through radiation-driven winds, a process enhanced by higher metallicity. In this channel, the surviving outer layers are helium-rich and hydrogen-deficient at the time of collapse, producing a Type Ib event when the core collapses.
Binary-channel channel: An alternative and increasingly favored pathway involves mass transfer in a close binary system. A hydrogen-rich primary can be stripped of its envelope by a companion, leaving a helium-rich core ready to explode as a Type Ib supernova. Binary evolution can extend the range of initial masses that produce stripped-envelope supernovae and helps explain events that occur in environments where single-star winds would be insufficient to remove the envelope.
Progenitor candidates: Potential progenitors include helium stars and Wolf-Rayet-like objects whose outer hydrogen layers have been peeled away. Direct detections of progenitors are challenging, but pre-explosion imaging and late-time observations have placed important constraints on the properties of the stars that give rise to Type Ib events.

Observational properties and demographics

Rates and environments: Type Ib supernovae constitute a minority of core-collapse supernovae, with their exact share depending on selection effects, metallicity, and star-formation activity. They are often found in star-forming galaxies, where massive stars are common, and their occurrence can reflect the metallicity and binarity statistics of the host stellar population.
Spectral diversity: While the hallmark is helium lines, the strength of those lines can vary among events. Some spectra may show weaker helium features, leading to classification ambiguity with certain Type Ic events. Non-thermal processes, ejecta mixing, and the viewing angle can influence the observed features.
Energetics and nucleosynthesis: Type Ib supernovae typically release energies on the order of a few times 10^51 ergs and synthesize radioactive nickel-56, which powers the late-time light curve. The detailed nucleosynthesis yields contribute to the chemical enrichment of galaxies, including iron-peak and intermediate-mass elements.

Relationships to other transients

Stripped-envelope family: Type Ib is closely related to Type Ic; together they form the broader category of stripped-envelope supernovae, which exclude the hydrogen-rich Type II events. The distinction hinges on the presence or absence of helium in the outer ejecta.
Connection to gamma-ray bursts: In a minority of cases, stripped-envelope supernovae are linked to long-duration gamma-ray bursts, most prominently associated with Type Ic events where the jet can escape the stellar envelope. Type Ib events are less commonly connected to GRBs, but the physics of jet formation and envelope stripping remains an active area of study.
Implications for stellar evolution: The existence and properties of Type Ib events inform models of massive-star evolution, mass loss mechanisms, and binary interactions, helping to constrain the relative importance of winds versus binary stripping in shaping the final fate of massive stars.

Controversies and ongoing debates

Progenitor identity: A central scientific question is how often Type Ib supernovae arise from single massive stars with strong winds versus binary-star systems with mass transfer. Observational evidence from pre-explosion imaging and late-time studies remains mixed, and theoretical models continue to refine the expected rates for each channel.
Helium line formation: The visibility of He I lines depends on ionization, non-thermal excitation, and the distribution of helium in the ejecta. Some events may appear helium-poor due to conditions that suppress He I line formation, leading to potential misclassification as Type Ic. This has implications for population statistics and the inferred channels of envelope stripping.
Metallicity effects: The efficiency of wind-driven hydrogen envelope loss in single stars is tied to metallicity. Debates persist about how metallicity biases the relative contributions of single-star and binary pathways across different galaxies and cosmic epochs.
Classification boundaries: The dividing line between Type Ib and Type Ic is not always clear-cut in practice. Spectroscopic observations near peak brightness can yield ambiguous results, and the evolving nature of spectra can blur distinctions, especially for distant events where data are sparser.
Progenitor detection challenges: Directly identifying progenitors is difficult due to observational limits and the rarity of nearby events with deep, pre-explosion imaging. This limits the ability to decisively confirm progenitor types and requires careful statistical treatment of non-detections and upper limits.