Zwicky Chemical Factory

Zwicky Transient Facility

The Zwicky Chemical Factory is powered by the Zwicky Transient Facility (ZTF). ZTF is named after Fritz Zwicky. ZTF is a time-domain survey (i.e., it regularly observes the same portions of the sky to discover celestial objects that are changing in brightness, moving, or exploding) that uses an ultra-wide-field camera to take very large pictures of the night sky. The camera is located on the 48-inch telescope on Palomar Observatory in southern California. The combination of the large field of view and time-domain observations allows ZTF to accomplish a lot of different science, such as the recent discovery of the closest known asteroid to ever pass Earth.

Every night ZTF looks at millions upon millions of galaxies, and finds hundreds of thousands of sources that have changed in brightness, and each of those sources is announced to the world. The night sky is always changing, and ZTF is there to capture it.

Bright Transient Survey

The Bright Transient Survey (BTS) is a sub-survey within ZTF designed to obtain a spectrum of every moderately bright supernova (SN) discovered by ZTF. Supernovae (SNe) are stellar explosions that are largely responsible for the creation of heavy elements throughout the universe.

The elements created by an individual SN differ depending on the type of SN (for example oxygen and sodium are primarily created when massive stars explode, whereas iron is primarily created when white dwarf stars explode). When ZTF discovers a new SN, all we know is that a star has exploded. We need to take a spectrum in order to identify the type of explosion, and hence the elements that have been created.

If we can manage to classify every SN found by ZTF, that will allow us to undertake a wide range of investigations to:

1. understand the origin of elements throughout the universe
2. understand how massive stars end their lives
3. understand how binary stars interact, and why some explode (and others don't)
4. understand why and how galaxies stop forming new stars

Supernovae

A supernova (SN) is a stellar explosion that emits an enormous amount of light. For thousands of years observers have looked up at the night sky and discovered novae (latin for "new star"). We now know that most of these novae were not in fact new stars, but instead small explosions on the surface of stars within the Milky Way galaxy that were otherwise too faint to be seen. In the early twentieth century, Fritz Zwicky and Walter Baade realized entire stars can explode, release a huge amount of energy and shine extremely bright, hence the name "super"-nova. Supernovae are so bright that they can be observed in distant galaxies across the universe; in some instances they shine brighter than all the other stars in their host galaxy combined.

Supernovae provide a death knell for massive stars and white dwarf stars, and they play an important role in the evolution of the universe. Many of the heavy elements in the universe are created in supernovae (for example, oxygen and iron). Supernovae both mix and expel the gas present in their host galaxies, which has important consequences for the subsequent formation of stars and planets. Finally, supernovae can be used to measure very precise distances, which led to the discovery of the accelerating universe powered by Dark Energy. This work was awarded the 2011 Nobel Prize in Physics.

While the ZTF camera is highly efficient at finding supernovae, it cannot, by itself, determine what type of supernova has been found. Other telescopes are needed to determine the type of supernova.

What is a Spectrum?

image credit: Astronomy by OpenStax is licensed under CC BY 4.0

An astronomical spectrum takes the light emitted by a celestial object, and breaks it up into its constituent wavelengths (much like passing the light of the Sun through a prism of glass). Obtaining the spectrum of any astronomical object, including supernovae, provides a great deal of information that we otherwise would not know. The reason for this is that individual atoms can only emit or absorb light at specific wavelengths. Spectra therefore tell us about chemical composition of what we are observing. In the case of supernovae, spectra also tell us how fast the ejected material is moving, by measuring the blueshift of the elements that are present.

What Can We Learn From a SN Spectrum?

"Spectrum is Truth." – Bob Kirschner

Supernova spectra contain a series of emission and absorption lines that tell us a lot about the star that has exploded. The lines are typically "broad" and the reason for this is that the material (i.e., gas) expelled during the explosion is hurtling away from the site of the explosion at very high speeds, typically more than 10,000 km per s (!). The spectrum also includes information about the chemical elements present in the ejected material.

For example, a Type Ia supernova spectrum is shown below:

Type Ia supernova spectra do not show hydrogen (which all stars have at the time of their birth), meaning these explosions must come from stars that have lost their hydrogren (in the case of type Ia supernovae, the exploding star is a white dwarf that is primarily made of carbon and oxygen).

By revealing which elements are present in the blast, SN spectra allow us to learn more about the star that has exploded.

Why Do SNe Have Absorption and Emission Lines?

Most of the example spectra that we show in the Zwicky Chemical Factory have absorption lines highlighted in order to determine the SN type. However, some SNe have prominent emission lines, while still others feature both. Below is an example Type II supernova, which shows hydrogen in both emission and absorption:

This "feature" in the spectrum is known as a P Cygni profile, named for the famous star P Cygni. In short, the light that is generated by a supernova explosion comes from the "center" of the explosion, and it must pass through the expanding ejecta. The gas that is moving towards us absorbs some of the light, and then re-radiates it in a random direction so that we can no longer see it. This leads to blueshifted absorption lines. Meanwhile, the gas moving away from us absorbs light that we would otherwise never see, and re-radiates it towards us. This creates the redshifted emission lines.

Most SN spectra are dominated by absorption lines, but some (especially some Type II supernovae) only show emission lines.

How Are "Narrow" Emission Lines Created?

The width of an absorption or emission line is determined by the speed at which the gas is moving. SN lines are "broad" because the gas is moving at very high speeds. "Narrow" lines come from gas that is not moving very much. SNe with "narrow" emission lines (such as type IIn or type Ibn SNe) have expelled slow-moving gas in the centuries and millenia before they exploded, and it is this gas that is the source of the narrow lines.

One of the major challenges in determining a SN type is when there are narrow lines present. The reason for this is that it can be difficult to separate whether or not the narrow lines are from the galaxy or the SN. For example, here is a galaxy spectrum:

While we do our best to remove light from the galaxy when studying SNe, it isn't always possible. A SN in the galaxy above might appear to have narrow hydrogen emission. Typically, SNe with narrow H emission also have a bit of broad H emission as well (so if there are only narrow lines that may be a hint they are only from the galaxy). If you find particularly difficult examples, provide your best estimate of the SN type, and then flag the source in the talk section.

SN Classification

One of the pressing challenges for the BTS is classifying the SN spectra that we obtain. (This is why we need your help!) We have algorithms that do a decent job of classifying spectra, but there are subtle differences between some SN types and the automated algorithms cannot always tell the difference.

There are 3 main types of supernova that we are trying to identify:

• type Ia supernovae – no hydrogen
• type II supernovae – has hydrogen
• type Ib/c supernovae – no hydrogen; has helium (Ib); no helium (Ic)

In the Zwicky Chemical Factory we are asking you to help classify these spectra. On average, we observe 3 or 4 new supernovae every night, so there are new spectra to look at nearly every single day. Every once and a while we also manage to capture a truly spectacular explosion.

Type Ia SNe

Type Ia supernovae are thermonuclear explosions of white dwarf stars that display strong absorption from silicon and calcium. Type Ia supernovae do not have either hydrogen or helium absorption. Sulfur and iron absorption can also be seen depending on the phase of the observations.

SNID Types Ia-norm, Ia-91T, Ia-91bg, Ia-02cx, Ia-csm, Ia-03fg

Type II SNe

Type II supernovae are the result of the core collapse phenomenon in massive stars in which the hydrogen envelope is still intact. These explosions show absorption from hydrogen. The strength of the hydrogen absorption depends on the SN subtype and the phase of the observations.

SNID Types II-norm, IIb, IIn

Type Ib/c SNe

Type Ib/c supernovae are the result of the core collapse phenomenon in massive stars that have no hydrogen. These explosions lack absorption from hydrogen. Some show absorption from helium (SNe Ib) while others lack absorption from both hydrogen and helium (SNe Ic).

SNID Types Ib, Ic, Ibn, Ic-BL, Ic-SLSN

What is SNID?

Once we obtain a supernova spectrum, we attempt to classify it using the SuperNova IDentification (SNID) algorithm. SNID has a large "bank" of template spectra (historical supernovae that are really well studied and understood). For any new spectrum, SNID attempts to automatically find the closest match in the template bank. Most of the time SNID is correct, but the differences between SN types can often be subtle and this is why we need your help. While SNID can do a lot of the work, we need humans to verify the final classifications.