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  • richardmitnick 3:49 pm on September 29, 2021 Permalink | Reply
    Tags: "Investigating the potential for life around the galaxy’s smallest stars", , , , , M Dwarf stars, The star at the center of the study is an M-dwarf called L 98-59.,   

    From University of California-Riverside (US) : “Investigating the potential for life around the galaxy’s smallest stars” 

    UC Riverside bloc

    From University of California-Riverside (US)

    September 29, 2021
    Jules L Bernstein
    Senior Public Information Officer
    (951) 827-4580
    jules.bernstein@ucr.edu

    1
    Artist rendering of an M-dwarf star, with three exoplanets orbiting. About 75 percent of all stars in the sky are the cooler, smaller red dwarfs. (NASA)

    New telescope will see planetary neighbors’ atmospheres.

    When the world’s most powerful telescope launches into space this year, scientists will learn whether Earth-sized planets in our ‘solar neighborhood’ have a key prerequisite for life — an atmosphere.

    National Aeronautics Space Agency(USA)/European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/ Canadian Space Agency [Agence Spatiale Canadienne](CA) Webb Infrared Space Telescope(US) James Webb Space Telescope annotated. Scheduled for launch in October 2021.

    These planets orbit an M-dwarf, the smallest and most common type of star in the galaxy. Scientists do not currently know how common it is for Earth-like planets around this type of star to have characteristics that would make them habitable.

    “As a starting place, it is important to know whether small, rocky planets orbiting M-dwarfs have atmospheres,” said Daria Pidhorodetska, a doctoral student in UC Riverside’s Department of Earth and Planetary Sciences. “If so, it opens up our search for life outside our solar system.”

    To help fill this gap in understanding, Pidhorodetska and her team studied whether the soon-to-launch James Webb Space Telescope, or the currently-in-orbit Hubble Space Telescope, are capable of detecting atmospheres on these planets. They also modeled the types of atmospheres likely to be found, if they exist, and how they could be distinguished from each other. The study has now been published in The Astronomical Journal.

    Study co-authors include astrobiologists Edward Schwieterman and Stephen Kane from UCR, as well as scientists from Johns Hopkins University (US), NASA’s Goddard Space Flight Center (US), Cornell University (US) and The University of Chicago (US).

    The star at the center of the study is an M-dwarf called L 98-59, which measures only 8% of our sun’s mass. Though small, it is only 35 light years from Earth. It’s brightness and relative closeness make it an ideal target for observation.

    Shortly after they form, M-dwarfs go through a phase in which they can shine two orders of magnitude brighter than normal. Strong ultraviolet radiation during this phase has the potential to dry out their orbiting planets, evaporating any water from the surface and destroying many gases in the atmosphere.

    “We wanted to know if the ablation was complete in the case of the two rocky planets, or if those terrestrial worlds were able to replenish their atmospheres,” Pidhorodetska said.

    The researchers modeled four different atmospheric scenarios: one in which the L 98-59 worlds are dominated by water, one in which the atmosphere is mainly composed of hydrogen, a Venus-like carbon dioxide atmosphere, and one in which the hydrogen in the atmosphere escaped into space, leaving behind only oxygen and ozone.

    They found that the two telescopes could offer complementary information using transit observations, which measure a dip in light that occurs as a planet passes in front of its star. The L 98-59 planets are much closer to their star than Earth is to the sun. They complete their orbits in less than a week, making transit observations by telescope faster and more cost effective than observing other systems in which the planets are farther from their stars.

    “It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds,” Schwieterman said. “With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”

    Of the four atmospheric scenarios the researchers considered, Pidhorodetska said the dried-out oxygen-dominated atmosphere is the most likely.

    “The amount of radiation these planets are getting at that distance from the star is intense,” she said.

    Though they may not have atmospheres that lend themselves to life today, these planets can offer an important glimpse into what might happen to Earth under different conditions, and what might be possible on Earth-like worlds elsewhere in the galaxy.

    The L 98-59 system was only discovered in 2019, and Pidhorodetska said she is excited to get more information about it when Webb is launched later this year.

    “We’re on the precipice of revealing the secrets of a star system that was hidden until very recently,” Pidhorodetska said.

    See the full article here .

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    University of California-Riverside Campus

    The University of California-Riverside (US) is a public land-grant research university in Riverside, California. It is one of the 10 campuses of the University of California (US) system. The main campus sits on 1,900 acres (769 ha) in a suburban district of Riverside with a branch campus of 20 acres (8 ha) in Palm Desert. In 1907, the predecessor to UC-Riverside was founded as the UC Citrus Experiment Station, Riverside which pioneered research in biological pest control and the use of growth regulators responsible for extending the citrus growing season in California from four to nine months. Some of the world’s most important research collections on citrus diversity and entomology, as well as science fiction and photography, are located at Riverside.

    The University of California-Riverside’s undergraduate College of Letters and Science opened in 1954. The Regents of the University of California declared UC-Riverside a general campus of the system in 1959, and graduate students were admitted in 1961. To accommodate an enrollment of 21,000 students by 2015, more than $730 million has been invested in new construction projects since 1999. Preliminary accreditation of the UC-Riverside School of Medicine was granted in October 2012 and the first class of 50 students was enrolled in August 2013. It is the first new research-based public medical school in 40 years.

    University of California-Riverside is classified among “R1: Doctoral Universities – Very high research activity.” The 2019 U.S. News & World Report Best Colleges rankings places University of California-Riverside tied for 35th among top public universities and ranks 85th nationwide. Over 27 of University of California-Riverside’s academic programs, including the Graduate School of Education and the Bourns College of Engineering, are highly ranked nationally based on peer assessment, student selectivity, financial resources, and other factors. Washington Monthly ranked University of California-Riverside 2nd in the United States in terms of social mobility, research and community service, while U.S. News ranks University of California-Riverside as the fifth most ethnically diverse and, by the number of undergraduates receiving Pell Grants (42 percent), the 15th most economically diverse student body in the nation. Over 70% of all University of California-Riverside students graduate within six years without regard to economic disparity. University of California-Riverside’s extensive outreach and retention programs have contributed to its reputation as a “university of choice” for minority students. In 2005, University of California-Riverside became the first public university campus in the nation to offer a gender-neutral housing option. University of California-Riverside’s sports teams are known as the Highlanders and play in the Big West Conference of the National Collegiate Athletic Association (NCAA) Division I. Their nickname was inspired by the high altitude of the campus, which lies on the foothills of Box Springs Mountain. The University of California-Riverside women’s basketball team won back-to-back Big West championships in 2006 and 2007. In 2007, the men’s baseball team won its first conference championship and advanced to the regionals for the second time since the university moved to Division I in 2001.

    History

    At the turn of the 20th century, Southern California was a major producer of citrus, the region’s primary agricultural export. The industry developed from the country’s first navel orange trees, planted in Riverside in 1873. Lobbied by the citrus industry, the University of California Regents established the UC Citrus Experiment Station (CES) on February 14, 1907, on 23 acres (9 ha) of land on the east slope of Mount Rubidoux in Riverside. The station conducted experiments in fertilization, irrigation and crop improvement. In 1917, the station was moved to a larger site, 475 acres (192 ha) near Box Springs Mountain.

    The 1944 passage of the GI Bill during World War II set in motion a rise in college enrollments that necessitated an expansion of the state university system in California. A local group of citrus growers and civic leaders, including many University of California-Berkeley(US) alumni, lobbied aggressively for a University of California-administered liberal arts college next to the CES. State Senator Nelson S. Dilworth authored Senate Bill 512 (1949) which former Assemblyman Philip L. Boyd and Assemblyman John Babbage (both of Riverside) were instrumental in shepherding through the State Legislature. Governor Earl Warren signed the bill in 1949, allocating $2 million for initial campus construction.

    Gordon S. Watkins, dean of the College of Letters and Science at University of California-Los Angeles, became the first provost of the new college at Riverside. Initially conceived of as a small college devoted to the liberal arts, he ordered the campus built for a maximum of 1,500 students and recruited many young junior faculty to fill teaching positions. He presided at its opening with 65 faculty and 127 students on February 14, 1954, remarking, “Never have so few been taught by so many.”

    University of California-Riverside’s enrollment exceeded 1,000 students by the time Clark Kerr became president of the University of California system in 1958. Anticipating a “tidal wave” in enrollment growth required by the baby boom generation, Kerr developed the California Master Plan for Higher Education and the Regents designated Riverside a general university campus in 1959. University of California-Riverside’s first chancellor, Herman Theodore Spieth, oversaw the beginnings of the school’s transition to a full university and its expansion to a capacity of 5,000 students. University of California-Riverside’s second chancellor, Ivan Hinderaker led the campus through the era of the free speech movement and kept student protests peaceful in Riverside. According to a 1998 interview with Hinderaker, the city of Riverside received negative press coverage for smog after the mayor asked Governor Ronald Reagan to declare the South Coast Air Basin a disaster area in 1971; subsequent student enrollment declined by up to 25% through 1979. Hinderaker’s development of innovative programs in business administration and biomedical sciences created incentive for enough students to enroll at University of California-Riverside to keep the campus open.

    In the 1990s, the University of California-Riverside experienced a new surge of enrollment applications, now known as “Tidal Wave II”. The Regents targeted University of California-Riverside for an annual growth rate of 6.3%, the fastest in the University of California system, and anticipated 19,900 students at University of California-Riverside by 2010. By 1995, African American, American Indian, and Latino student enrollments accounted for 30% of the University of California-Riverside student body, the highest proportion of any University of California campus at the time. The 1997 implementation of Proposition 209—which banned the use of affirmative action by state agencies—reduced the ethnic diversity at the more selective UC campuses but further increased it at University of California-Riverside.

    With University of California-Riverside scheduled for dramatic population growth, efforts have been made to increase its popular and academic recognition. The students voted for a fee increase to move University of California-Riverside athletics into NCAA Division I standing in 1998. In the 1990s, proposals were made to establish a law school, a medical school, and a school of public policy at University of California-Riverside, with the University of California-Riverside School of Medicine and the School of Public Policy becoming reality in 2012. In June 2006, University of California-Riverside received its largest gift, 15.5 million from two local couples, in trust towards building its medical school. The Regents formally approved University of California-Riverside’s medical school proposal in 2006. Upon its completion in 2013, it was the first new medical school built in California in 40 years.

    Academics

    As a campus of the University of California(US) system, University of California-Riverside is governed by a Board of Regents and administered by a president. University of California-Riverside’s academic policies are set by its Academic Senate, a legislative body composed of all University of California-Riverside faculty members.

    University of California-Riverside is organized into three academic colleges, two professional schools, and two graduate schools. University of California-Riverside’s liberal arts college, the College of Humanities, Arts and Social Sciences, was founded in 1954, and began accepting graduate students in 1960. The College of Natural and Agricultural Sciences, founded in 1960, incorporated the CES as part of the first research-oriented institution at University of California-Riverside; it eventually also incorporated the natural science departments formerly associated with the liberal arts college to form its present structure in 1974. University of California-Riverside’s newest academic unit, the Bourns College of Engineering, was founded in 1989. Comprising the professional schools are the Graduate School of Education, founded in 1968, and the University of California-Riverside School of Business, founded in 1970. These units collectively provide 81 majors and 52 minors, 48 master’s degree programs, and 42 Doctor of Philosophy (PhD) programs. University of California-Riverside is the only University of California campus to offer undergraduate degrees in creative writing and public policy and one of three University of California campuses(along with University of California-Berkeley (US) and University of California-Irvine (US)) to offer an undergraduate degree in business administration. Through its Division of Biomedical Sciences, founded in 1974, University of California-Riverside offers the Thomas Haider medical degree program in collaboration with University of California-Los Angeles(US). University of California-Riverside’s doctoral program in the emerging field of dance theory, founded in 1992, was the first program of its kind in the United States, and University of California-Riverside’s minor in lesbian, gay and bisexual studies, established in 1996, was the first undergraduate program of its kind in the University of California system. A new BA program in bagpipes was inaugurated in 2007.

    Research and economic impact

    University of California-Riverside operated under a $727 million budget in fiscal year 2014–15. The state government provided $214 million, student fees accounted for $224 million and $100 million came from contracts and grants. Private support and other sources accounted for the remaining $189 million. Overall, monies spent at University of California-Riverside have an economic impact of nearly $1 billion in California. University of California-Riverside research expenditure in FY 2018 totaled $167.8 million. Total research expenditures at University of California-Riverside are significantly concentrated in agricultural science, accounting for 53% of total research expenditures spent by the university in 2002. Top research centers by expenditure, as measured in 2002, include the Agricultural Experiment Station; the Center for Environmental Research and Technology; the Center for Bibliographical Studies; the Air Pollution Research Center; and the Institute of Geophysics and Planetary Physics.

    Throughout University of California-Riverside’s history, researchers have developed more than 40 new citrus varieties and invented new techniques to help the $960 million-a-year California citrus industry fight pests and diseases. In 1927, entomologists at the CES introduced two wasps from Australia as natural enemies of a major citrus pest, the citrophilus mealybug, saving growers in Orange County $1 million in annual losses. This event was pivotal in establishing biological control as a practical means of reducing pest populations. In 1963, plant physiologist Charles Coggins proved that application of gibberellic acid allows fruit to remain on citrus trees for extended periods. The ultimate result of his work, which continued through the 1980s, was the extension of the citrus-growing season in California from four to nine months. In 1980, University of California-Riverside released the Oroblanco grapefruit, its first patented citrus variety. Since then, the citrus breeding program has released other varieties such as the Melogold grapefruit, the Gold Nugget mandarin (or tangerine), and others that have yet to be given trademark names.

    To assist entrepreneurs in developing new products, University of California-Riverside is a primary partner in the Riverside Regional Technology Park, which includes the City of Riverside and the County of Riverside. It also administers six reserves of the University of California Natural Reserve System. UC-Riverside recently announced a partnership with China Agricultural University[中国农业大学](CN) to launch a new center in Beijing, which will study ways to respond to the country’s growing environmental issues. University of California-Riverside can also boast the birthplace of two name reactions in organic chemistry, the Castro-Stephens coupling and the Midland Alpine Borane Reduction.

     
  • richardmitnick 4:40 pm on July 4, 2017 Permalink | Reply
    Tags: , , , , , M Dwarf stars   

    From Centauri Dreams: “M-Dwarf Habitability: New Work on Flares” 

    Centauri Dreams

    July 4, 2017
    Paul Gilster

    The prospects for life around M-dwarf stars, always waxing and waning depending on current research, have dimmed again with the release of new work [Accepted in ApJ]from Christina Kay (NASA/GSFC) and colleagues. As presented at the National Astronomy Meeting at the University of Hull (UK), the study takes on the question of space weather and its effect on habitability.

    We know that strong solar flares can disrupt satellites and ground equipment right here on Earth. But habitable planets around M-dwarfs — with liquid water on the surface — must orbit far closer to their star than we do. Proxima Centauri b, for example, is roughly 0.05 AU from its small red host (7,500,000 km), while all seven of the TRAPPIST-1 planets orbit much closer than Mercury orbits the Sun. What, then, could significant flare activity do to such vulnerable worlds?

    Centauris Alpha Beta Proxima 27, February 2012. Skatebiker

    ESO Red Dots Campaign

    ESO Pale Red Dot project

    The TRAPPIST-1 star, an ultracool dwarf, is orbited by seven Earth-size planets (NASA).

    ESO Belgian robotic Trappist National Telescope at Cerro La Silla, Chile interior

    ESO Belgian robotic Trappist-South National Telescope at Cerro La Silla, Chile

    1
    Artist’s impression of HD 189733b, showing the planet’s atmosphere being stripped by the radiation from its parent star. Credit: Ron Miller.

    Working with Merav Opher and Marc Kornbleuth (both at Boston University), Kay has homed in on coronal mass ejections (CMEs), the vast upheavals that throw stellar plasma into nearby space. While red dwarf stars are significantly cooler than our G-class Sun, their CMEs are thought to be far stronger because of their enhanced magnetic fields. From the paper:

    “Stellar activity tends to increase with the size of the stellar convection envelope (West et al. 2004) and stellar rotation rates (Mohanty et al. 2002; West et al. 2015), although the activity saturates for sufficiently high rotational velocity (Delfosse et al. 1998). For mid- to late-type M dwarfs (M4 to M8.5) the activity saturates at higher rotational velocities than for early-type M dwarfs, and above M9 the activity levels decrease significantly (Mohanty et al. 2002). Accordingly, most M dwarf stars will have significantly enhanced stellar activity as compared to the Sun.”

    A strong planetary magnetic field could counteract at least some of the resulting flare activity, but even there a possibility of strong erosion of the atmosphere remains. And if the planet is tidally locked, some recent work suggests little to no magnetic field can be expected.

    The paper outlines the process when a CME hits a nearby planet. One problem is extreme ultraviolet and X-ray flux (XUV) which can heat the upper atmosphere and perhaps ionize it. If such radiation gets through to the surface, it can damage any potential life-forms there. While it turns out that an M-dwarf habitable zone planet receives an order of magnitude less XUV flux than Earth when the star is quiet, the flux jumps as high as 100 times Earth’s during the star’s frequent flare activity.

    Significant CME activity also makes the planet much less likely to retain its atmosphere as a shield for life on the surface. A CME compresses whatever magnetosphere the planet has, and in extreme cases, say the authors, can exert enough pressure to shrink the magnetosphere to the point where the atmosphere can be seriously eroded.

    Modeling an Astrospheric Current Sheet

    Kay’s team modeled the effects of theoretical CMEs on the red dwarf V374 Pegasi, using a tool Kay developed for CME modeling called ForeCAT. They found that the strong magnetic fields of the star produce CMEs that can reach the so-called Astrospheric Current Sheet, where the background magnetic field is at its minimum. The same effect occurs with our Sun, when solar CMEs are deflected by magnetic forces toward the minimum magnetic energy.

    At the Sun, the Heliospheric Current Sheet — the local analog to a different star’s Astrospheric Current Sheet — is a field that extends along the Sun’s equatorial plane in the heliosphere and is shaped by the effect of the Sun’s rotating magnetic field on the plasma in the solar wind. The HCS separates regions of the solar wind where the magnetic field points toward or away from the Sun.

    Let’s dwell on that for a moment. Here’s what a NASA fact sheet has to say about the Heliospheric Current Sheet:

    “The sun’s magnetic field permeates the entire solar system called the heliosphere. All nine planets orbit inside it. But the biggest thing in the heliosphere is not a planet, or even the sun. It’s the current sheet — a sprawling surface where the polarity of the sun’s magnetic field changes from plus (north) to minus (south). A small electrical current flows within the sheet, about 10−10 A/m². The thickness of the current sheet is about 10,000 km near the orbit of the Earth. Due to the tilt of the magnetic axis in relation to the axis of rotation of the sun, the heliospheric current sheet flaps like a flag in the wind. The flapping current sheet separates regions of oppositely pointing magnetic field, called sectors.”

    3
    The Heliospheric Current Sheet results from the influence of the Sun’s rotating magnetic field on the plasma in the interplanetary medium (solar wind). The wavy spiral shape has been likened to a ballerina’s skirt. The new work uses a software modeling package called ForeCAT to study interactions between CMEs and the Astrospheric Current Sheet around the red dwarf V374 Pegasi. Credit: NASA GSFC.

    Kay and team have modeled the Astrospheric Current Sheet expected to be found around M-dwarfs like V374 Pegasi. The authors find that upon reaching the ACS, CMEs become ‘trapped’ along it. Planets can dip into and out of the ACS as they orbit. A CME moving out into the Astrospheric Current Sheet around an M-dwarf can cancel out a habitable zone planet’s local magnetic field, opening the world to devastating flare effects. The upshot:

    “We expect that rocky exoplanets cannot generate sufficient magnetic field to shield their atmosphere from mid-type M dwarf CMEs… We expect that the minimum magnetic field strength will change with M dwarf spectral type as the amount of stellar activity and stellar magnetic field strength change, and that early-type M dwarfs would be more likely to retain an atmosphere than mid or late-type M dwarfs.”

    The authors calculate that a mid-type M-dwarf planet would need a minimum planetary magnetic field between tens to hundreds of Gauss to retain an atmosphere, values that are far higher than Earth’s (0.25 to 0.65 gauss). CME impacts as numerous as five per day could occur for planets near the star’s Astrospheric Current Sheet. The only mitigating factor is that the rate decreases for planets in inclined orbits. The paper notes:

    “The sensitivity to the inclination is much greater for the mid-type M dwarf exoplanets due to the extreme deflections to the Astrospheric Current Sheet. For low inclinations we find a probability of 10% whereas the probability decreases to 1% for high inclinations. From our estimation of 50 CMEs per day, we expect habitable mid-type M dwarf exoplanets to be impacted 0.5 to 5 times per day, 2 to 20 times the average at Earth during solar maximum. The frequency of CME impacts may have significant implications for exoplanet habitability if the impacts compress the planetary magnetosphere leading to atmospheric erosion.”

    So we have much to learn about M-dwarfs. In particular, how accurate is the ForeCAT model in developing the CME scenario around such stars? As we examine such modeling, we have to keep in mind that magnetic field strength will change with the type of M-dwarf we are dealing with. Based on this research, only early M-dwarfs are likely to maintain an atmosphere.

    The paper is Kay, Opher and Kornbleuth, Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars.

    Centauri Dreams

    See the full article here .

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    Tracking Research into Deep Space Exploration

    Alpha Centauri and other nearby stars seem impossible destinations not just for manned missions but even for robotic probes like Cassini or Galileo. Nonetheless, serious work on propulsion, communications, long-life electronics and spacecraft autonomy continues at NASA, ESA and many other venues, some in academia, some in private industry. The goal of reaching the stars is a distant one and the work remains low-key, but fascinating ideas continue to emerge. This site will track current research. I’ll also throw in the occasional musing about the literary and cultural implications of interstellar flight. Ultimately, the challenge may be as much philosophical as technological: to reassert the value of the long haul in a time of jittery short-term thinking.

     
  • richardmitnick 12:01 pm on April 20, 2016 Permalink | Reply
    Tags: , , , , M Dwarf stars   

    From AAS NOVA: “Choosing Stars to Search for Habitable Planets” 

    AASNOVA

    Amercan Astronomical Society

    20 April 2016
    Susanna Kohler

    1
    Artist’s illustration of an M-dwarf star surrounded by three planets. A recent study examines which stars make the best targets when searching for habitable exoplanets. [NASA/JPL-Caltech]

    M-dwarf stars are excellent targets for planet searches because the signal of an orbiting planet is relatively larger (and therefore easier to detect!) around small, dim M dwarfs, compared to Sun-like stars. But are there better or worse stars to target within this category when searching for habitable, Earth-like planets?
    Confusing the Signal

    Radial velocity campaigns search for planets by looking for signatures in a star’s spectra that indicate the star is “wobbling” due to the gravitational pull of an orbiting planet. Unfortunately, stellar activity can mimic the signal of an orbiting planet in a star’s spectrum — something that is particularly problematic for M dwarfs, which can remain magnetically active for billions of years. To successfully detect planets that orbit in their stars’ habitable zones, we have to account for this problem.

    In a recent study led by Elisabeth Newton (Harvard-Smithsonian Center for Astrophysics), the authors use literature measurements to examine the rotation periods for main-sequence, M-type stars. They focus on three factors that are important for detecting and characterizing habitable planets around M dwarfs:

    Whether the habitable-zone orbital periods coincide with the stellar rotation
    False planet detections caused by stellar activity often appear as a “planet” with an orbital period that’s a multiple of the stellar rotation period. If a star’s rotation period coincides with the range of orbital periods corresponding to its habitable zone, it’s therefore possible to obtain false detections of habitable planets.
    How long stellar activity and rapid rotation last in the star
    All stars become less magnetically active and rotate more slowly as they age, but the rate of this decay depends on their mass: lower-mass stars stay magnetically active for longer and take longer to spin down.
    Whether detailed atmospheric characterization will be possible
    It’s ideal to be able to follow up on potentially habitable exoplanets, and search for biosignatures such as oxygen in the planetary atmosphere. This type of detection will only be feasible for low-mass dwarfs, however, due to the relative size of the star and the planet.

    1
    Stellar rotation period as a function of stellar mass. The blue shaded region shows the habitable zone as a function of stellar mass. For M dwarfs between ~0.25 and ~0.5 solar mass, the habitable-zone period overlaps with the stellar rotation period. [Newton et al. 2016]

    An Ideal Range

    Newton and collaborators find that stars in the mass range of 0.25 to 0.5 solar mass (stellar class M1V-M4V) are non-ideal targets, because their stellar rotation periods (or a multiple thereof) coincide with the orbital periods of their habitable zones. In addition, atmospheric characterization will only be feasible in the near future for stars with mass less than ~0.25 solar mass.

    On the other hand, dwarfs with mass less than ~0.1 solar masses (stellar classes later than M6V) will retain their stellar activity and faster rotation rates throughout most of their lifetimes, making them non-ideal targets as well.

    When searching for habitable exoplanets, the best targets are therefore the mid M dwarfs in the mass range of 0.1 to 0.25 solar mass (stellar class M4V-M6V). Building a sample focused on these stars will reduce the likelihood that planets found in the stars’ habitable zones are false detections. This will hopefully produce a catalog of potentially habitable exoplanets that we can eventually follow up with atmospheric observations.
    Citation

    Elisabeth R. Newton et al 2016 ApJ 821 L19. doi:10.3847/2041-8205/821/1/L19

    Science paper:
    THE IMPACT OF STELLAR ROTATION ON THE DETECTABILITY OF HABITABLE PLANETS AROUND M DWARFS

    See the full article here .

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  • richardmitnick 12:55 pm on December 4, 2015 Permalink | Reply
    Tags: , , , M Dwarf stars   

    From AAS NOVA: “Preferred Hosts for Short-Period Exoplanets” 

    AASNOVA

    Amercan Astronomical Society

    4 December 2015
    Susanna Kohler

    1
    Illustration of planets orbiting an M-dwarf star. A new study finds that M dwarfs host 3.5 times the number of small planets as do larger, main-sequence stars. [NASA/JPL-Caltech]

    In an effort to learn more about how planets form around their host stars, a team of scientists has analyzed the population of Kepler-discovered exoplanet candidates, looking for trends in where they’re found.

    NASA Kepler Telescope
    Kepler

    Planetary Occurrence

    Since its launch in 2009, Kepler has found thousands of candidate exoplanets around a variety of star types. Especially intriguing is the large population of “super-Earths” and “mini-Neptunes” — planets with masses between that of Earth and Neptune — that have short orbital periods. How did they come to exist so close to their host star? Did they form in situ, or migrate inwards, or some combination of both processes?

    To constrain these formation mechanisms, a team of scientists led by Gijs Mulders (University of Arizona and NASA’s NExSS coalition) analyzed the population of Kepler planet candidates that have orbital periods between 2 and 50 days.

    Mulders and collaborators used statistical reconstructions to find the average number of planets, within this orbital range, around each star in the Kepler field. They then determined how this planet occurrence rate changed for different spectral types — and therefore the masses — of the host stars: do low-mass M-dwarf stars host more or fewer planets than higher-mass, main-sequence F, G, or K stars?

    Challenging Models

    2
    Authors’ estimates for the occurrence rate for short-period planets of different radii around M-dwarfs (purple) and around F, G, and K-type stars (blue). [Mulders et al. 2015]

    The team found that M dwarfs, compared to F, G, or K stars, host about half as many large planets with orbital periods of P < 50 days. But, surprisingly, they host significantly more small planets, racking up an average of 3.5 times the number of planets in the size range of 1–2.8 Earth-radii.

    Could it be that M dwarfs have a lower total mass of planets, but that mass is distributed into more, smaller planets? Apparently not: the authors show that the mass of heavy elements trapped in short-orbital-period planets is higher for M dwarfs than for the larger F, G and K stars.

    All of this goes contrary to expectation, because we know that protostellar disks, from which planets form, are more massive around larger-mass stars. So why is there more heavy-element mass trapped in planetary systems with low stellar mass?

    This outcome isn’t predicted by either in situ or migration planet formation theories. The authors instead propose that the distribution could be explained if the inward drift of planetary building blocks — either dust grains or protoplanets — turns out to be more efficient around lower-mass stars.

    Citation

    Gijs D. Mulders et al 2015 ApJ 814 130. doi:10.1088/0004-637X/814/2/130

    See the full article here .

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