Study of Low-Mass Companions to Stars

This study focuses on a group of 28 low-mass companions to stars that were observed by the Kepler mission. The Kepler mission was designed to find Earth-sized planets orbiting stars similar to the Sun. These companions, known as Kepler Objects of Interest (KOIs), are important for understanding how planets and stars form and evolve.

The companions we looked at include eight that have already been confirmed as planets. They were found by tracking how these objects cross in front of their host stars, which causes a dip in the star's brightness. As part of this research, we used a special telescope at the Apache Point Observatory, which helps us collect data more efficiently.

We gathered a lot of data from the APOGEE-N project, which collected thousands of spectra, or detailed light measurements, from many KOIs. This allowed us to learn more about the size and mass of these companions, helping to improve our understanding of their characteristics.

#Importance of Characterization

Being able to accurately determine the mass and radius of these companions is key to understanding their formation and evolution. Low-mass stars, including M dwarfs and Brown Dwarfs, are often targeted in research because they can have closer habitable zones. This means that planets in those zones can be easier to detect and study.

Getting these measurements right is essential for figuring out the properties of the planets themselves. If we want to understand what these planets are made of and how they behave, we need precise information about their host stars.

#Characteristics of Brown Dwarfs

Brown dwarfs are objects that are not quite stars and not quite planets. They exist in a mass range between the smallest stars and the largest planets. Understanding the formation of brown dwarfs and their relationship with their host stars is critical for learning about star and planet formation processes.

However, there aren't many brown dwarfs whose characteristics have been well defined. Tacking on additional well-characterized brown dwarfs to the existing samples can help scientists better understand their formation mechanisms.

#Survey Methodology

We present results from a specific survey of KOIs conducted through the Sloan Digital Sky Survey IV. This sample included brown dwarfs and fully convective M dwarf companions to Sun-like stars. We collected both photometric data (brightness measurements over time) and spectral data (information about the light's properties) to learn more about these objects.

The paper explains the methods used for collecting data, modeling the observations, and how we compared our findings with theoretical models.

#Kepler Mission Overview

The Kepler mission was launched to explore how many Earth-sized planets exist in the habitable zones of their stars. It gathered a large volume of data by monitoring the brightness of over 150,000 stars over four years. Its observations allow researchers to examine the frequency and characteristics of planets orbiting stars.

To confirm a planet's existence, researchers use various methods to rule out other explanations, such as false positives from other stellar phenomena. The validation process is critical in determining which signals represent actual planets.

#The APOGEE-KOI Program

This program aimed to improve understanding of the KOI companions by providing additional data through Spectroscopic Observations. The APOGEE project, which uses a high-resolution spectrograph, allows for precise measurements of Radial Velocities-essentially the speed at which an object moves toward or away from us. These measurements help to confirm the presence of companions and refine our knowledge of their properties.

The APOGEE-KOI program worked alongside Kepler's existing observations to provide valuable insight into previously observed candidates, confirming which ones were genuine planets and which were not.

#Data Collection Process

Data for the study were collected using a variety of instruments. The northern Apache Point Observatory (APO) allows multiple stars to be observed at once, increasing data collection efficiency. This facility has capabilities to observe many objects simultaneously, which is crucial for studying a large number of KOIs.

Overall, many targets were selected based on their brightness and proximity to the observatory. The selection criteria ensured that the data collected would have the necessary signal-to-noise ratio for accurate analysis.

#Observations with Kepler

Kepler observed the KOIs primarily in two modes: long cadence and short cadence. Long cadence captures data every 30 minutes, while short cadence does so every 2 minutes. The ability to gather data at these frequencies allows researchers to track the transit of planets across their host stars effectively.

Data quality is paramount, and systematic errors can occur due to factors such as cosmic rays or spacecraft events. Researchers rely on strict quality control measures to filter out bad data and focus on high-quality observations for analysis.

#Adaptive Optics Imaging

Adaptive optics imaging was used to obtain high-resolution images of some KOIs. The Robo-AO system at Kitt Peak National Observatory is designed to correct for atmospheric disturbances, leading to clearer images of distant stars. This allows researchers to check for potential nearby companions that could affect the data.

Robo-AO observations help ensure that the signals measured are not due to unrelated nearby objects, allowing for a clearer understanding of the KOIs.

#Doppler Spectroscopy

Doppler spectroscopy was a key technique used in this study. The method relies on measuring changes in the frequency of light waves emitted by the stars. By observing these changes, researchers can determine the speed of a star in relation to Earth, which helps confirm the presence of orbiting companions.

This technique proved effective in providing accurate measurements of radial velocities for 28 KOIs, allowing for a deeper understanding of their characteristics.

#SED and Photometric Modeling

Analyzing the Spectral Energy Distribution (SED) of star systems provides additional insights into their physical properties. By combining data from Kepler with APOGEE measurements, researchers created a model to estimate various parameters for the KOIs.

The models used factors like brightness and color to derive information about the mass, radius, and age of both the stars and their companions. This multi-layered approach helps improve accuracy for the observed characteristics.

#Findings on Brown Dwarfs

Among the 28 KOIs, five were identified as brown dwarfs. These less common objects were of particular interest because they shed light on the diversity of planetary companions. The characteristics of these brown dwarfs were further explored, updating previously known data and confirming their existence.

Researchers found that some brown dwarfs exhibited unique features typical of low-mass companions. This knowledge expands the database of known brown dwarfs and provides new avenues for exploration.

#Examining KOIs for Eccentricities

The study compared the eccentricities of brown dwarf-hosting systems to publicly available data on other known systems. Understanding the relationship between periods and eccentricities contributes to our knowledge of how these objects behave in their orbits.

Many KOIs showed expected patterns, with short-period systems generally showing lower eccentricities. Brown dwarfs were found to have similar traits to their counterparts in other studies, allowing for a better understanding of their orbital mechanics.

#Insights into Low-Mass Companions

Some of the KOIs had previously been categorized as genuine planets through statistical validation methods. However, using the APOGEE data, researchers could determine that they were, in fact, low-mass stellar companions.

These findings highlight the challenges researchers face in distinguishing between low-mass stars and planets. The results reveal that careful analysis is necessary to avoid misclassifications.

#Using Gaia Metrics

Gaia, another space mission, provides valuable data for follow-up studies. The re-normalized unit weight error (RUWE) is useful for detecting unresolved companion stars that may have influenced observations.

In this study, the majority of KOIs were found to have RUWE values suggesting they do not exhibit signs of having close companions. This reinforces the reliability of the findings, as most KOIs conform to a single-star model.

#Collaboration with Other Missions

The data obtained from APOGEE-N complements findings from high-resolution spectrographs used in other projects. These collaborations allow for better characterizations of the KOIs, as data from multiple sources provide a more comprehensive understanding.

The use of multiple instruments and missions strengthens the results, allowing researchers to draw conclusions with greater confidence.

#Summary of Findings

In summary, the analysis of the 28 KOIs has revealed important insights into the properties of low-mass companions. The results show that some companions are slightly larger than models predict, suggesting the potential influence of factors like magnetic activity.

By combining data from Kepler and APOGEE-N, researchers have derived critical physical parameters that enhance our understanding of these objects. Findings from this study will contribute to future research and provide a foundation for ongoing exploration of low-mass stars and their companions.

#Conclusion

This research sheds light on the diversity of companions orbiting Sun-like stars and their implications for our understanding of planet and star formation. By studying low-mass companions, we gain deeper insights into the broader landscape of exoplanets and their behavior.

These findings underscore the importance of continued exploration and collaboration among different missions. The pursuit of knowledge in this field remains vital for answering fundamental questions about the universe and our place within it.