Cormac Larkin
Career Stage
Student (undergraduate)
Poster Abstract
OB stars are the most massive and hottest stars, and are important for understanding cosmic reionization sources, stellar and intermediate mass black holes and gravitational wave sources. They are of particular interest in the SMC due to its lower metallicity, which is reminiscent of the early Universe and the Epoch of Reionization.
In this work, we combine ultraviolet data from the Swift/UVOT satellite with optical and infrared catalog data. After data pre-selection, we create spectral energy distributions, and then fit model stellar atmospheres to obtain good estimates of effective temperature. OB stars emit the majority of their radiation in the ultraviolet part of the spectrum, so the addition of this data helps to constrain these temperature estimates. By comparing the fitted temperatures to literature values of previously known objects, we aim to identify new candidate OB stars.
Plain text summary
Stars come in all shapes and sizes, and generally speaking, larger stars are hotter. The largest and hottest stars are known as “OB” stars, and they are interesting for a variety of reasons. In the early Universe, a process called reionization happened, at which time the first stars and galaxies formed. This process was caused by energy sources, and it’s thought that stars similar to the OB stars we see today played a role in this. When OB stars come to the end of their lives, they can form black holes under certain circumstances. If these black holes merge together, they can stretch space-time itself, causing gravitational waves. By studying OB stars, we hope to better understand these things as well.
The Small Magellanic Cloud (SMC) is a dwarf galaxy about 200,000 light-years away from our own Milky Way. The ratio of so-called “heavy elements” in the SMC compared to hydrogen and helium is only about 20% of the ratio in the Milky Way, so the environment is more similar to the early Universe during the reionization period (when there was only hydrogen and helium to begin with). The amount of heavy elements present when stars form determines their chemical makeup, so studying OB stars in the SMC, where the conditions are closer to those of the early Universe, is interesting.
In this work, we use data in the ultraviolet, optical and infrared parts of the spectrum. These data are magnitudes (brightnesses) at certain wavelengths. We first remove some data that have large errors or are too faint or cold to be OB stars. In the first figure, we show a color-magnitude diagram of the data, which is similar to a plot of brightness on the y-axis vs the temperature on the x-axis. On this plot we show the cutoff values for brightness and temperature that we used. We also plotted the color-magnitude values of the currently known OB stars in our area of coverage, from the Bonanos et al. 2010 catalog. The vast majority of these known stars are inside our cutoff values, which helps to justify the values we arrived at.
After this selection process, we then create what are known as Spectral Energy Distributions (SEDs) using the data. This is a plot of a star’s energy output versus wavelength. By fitting models to the SED, you can get a good estimate of the star’s surface temperature, which tells you a lot about what kind of star it might be. However, stars with very different temperatures can appear quite similar on an SED plot if the peak of the SED isn’t inside the wavelength range, since this peak occurs at a specific wavelength depending on the temperature. Since OB stars shine brightest in the ultraviolet region, this has been an issue. You can see this in the second figure of the poster, where sample SEDs for stars of 10, 20 and 40 thousand Kelvin are shown. They are quite similar in the optical and infrared region (except for an offset on the y-axis) but you can see differences in the ultraviolet region.
What we have done in this work is added ultraviolet data from NASA’s Swift satellite, and this additional data in the ultraviolet region makes model fitting more reliable. We can fit models to SEDs of stars where the temperature has already been determined by other means, and use these comparisons to verify possible new stars not already in the literature. We are working on this step at the moment, this is a work in progress!
The Small Magellanic Cloud (SMC) is a dwarf galaxy about 200,000 light-years away from our own Milky Way. The ratio of so-called “heavy elements” in the SMC compared to hydrogen and helium is only about 20% of the ratio in the Milky Way, so the environment is more similar to the early Universe during the reionization period (when there was only hydrogen and helium to begin with). The amount of heavy elements present when stars form determines their chemical makeup, so studying OB stars in the SMC, where the conditions are closer to those of the early Universe, is interesting.
In this work, we use data in the ultraviolet, optical and infrared parts of the spectrum. These data are magnitudes (brightnesses) at certain wavelengths. We first remove some data that have large errors or are too faint or cold to be OB stars. In the first figure, we show a color-magnitude diagram of the data, which is similar to a plot of brightness on the y-axis vs the temperature on the x-axis. On this plot we show the cutoff values for brightness and temperature that we used. We also plotted the color-magnitude values of the currently known OB stars in our area of coverage, from the Bonanos et al. 2010 catalog. The vast majority of these known stars are inside our cutoff values, which helps to justify the values we arrived at.
After this selection process, we then create what are known as Spectral Energy Distributions (SEDs) using the data. This is a plot of a star’s energy output versus wavelength. By fitting models to the SED, you can get a good estimate of the star’s surface temperature, which tells you a lot about what kind of star it might be. However, stars with very different temperatures can appear quite similar on an SED plot if the peak of the SED isn’t inside the wavelength range, since this peak occurs at a specific wavelength depending on the temperature. Since OB stars shine brightest in the ultraviolet region, this has been an issue. You can see this in the second figure of the poster, where sample SEDs for stars of 10, 20 and 40 thousand Kelvin are shown. They are quite similar in the optical and infrared region (except for an offset on the y-axis) but you can see differences in the ultraviolet region.
What we have done in this work is added ultraviolet data from NASA’s Swift satellite, and this additional data in the ultraviolet region makes model fitting more reliable. We can fit models to SEDs of stars where the temperature has already been determined by other means, and use these comparisons to verify possible new stars not already in the literature. We are working on this step at the moment, this is a work in progress!
Poster file
Poster Title
Applying UVOT data to finding new OB stars in the Small Magellanic Cloud
Url
Twitter: @LarkinCormac Email: larkin@astro.rug.nl