Ursa Major Group Research Articles

Kinematic structure of the stellar population of the solar neighborhood by Gaia DR3

The data of modern catalogs allow us to consider in detail the issue of the spatial velocity distribution. The stars in the solar vicinity make it possible to avoid errors and draw detailed conclusions about their kinematics. For a corrected study, we took a sample of stars from the Gaia DR3 within 300 pc. This area contains a number of star clusters and streams. It was found that in the circumscribed vicinity the direction of the stars is divided into several vast concentrations, the apex of motion of which lies in the direction of the motion of the Sun and in the direction of motion of the Ursa Major group, which in turn is divided into two separate groups in direction in space. In addition, a group of stars is considered, which, according to their kinematic characteristics, most likely belong to a halo or bulge. High-velocity stars are also considered under the assumption of their nature.

The Dynamics of Interstellar Asteroids and Comets within the Galaxy: An Assessment of Local Candidate Source Regions for 1I/’Oumuamua and 2I/Borisov

Abstract The low velocity of interstellar asteroid 1I/’Oumuamua with respect to our galaxy’s local standard of rest implies that it is young. Adopting the young age hypothesis, we assess possible origin systems for this interstellar asteroid and for 2I/Borisov, though the latter’s higher speed means it is unlikely to be young. First, their past trajectories are modeled under gravitational scattering by galactic components (“disk heating”) to assess how far back one can trace them. The stochastic nature of disk heating means that a back integration can only expect to be accurate to within 15 pc and 2 km s−1 at −10 Myr, dropping steeply to 400 pc and 10 km s−1 at −100 Myr, sharply limiting our ability to determine a precise origin. Nevertheless, we show that ’Oumuamua’s origin system likely is currently within 1 kpc of Earth, in the local Orion Arm. Second, we back integrate ’Oumuamua’s trajectory in order to assess source regions, emphasizing young systems and moving groups. Though disk heating allows for only a statistical link to source regions, ’Oumuamua passed through a considerable subset of the Carina and Columba moving groups when those groups were forming. This makes them perhaps the most plausible source region, if ’Oumuamua was ejected during planet formation or via intra-cluster interactions. We find three stars in the Ursa Major group, one brown dwarf, and seven other stars to have plausible encounters with 2I/Borisov, within 2 pc and 30 km s−1. These encounters’ high relative speeds mean none are likely to be the home of 2I/Borisov.

Coronagraphic search for wide substellar companions among members of the Ursa Major moving group

We present the results of a survey to detect low-mass companions of UMa group members, carried out in 2003-2006 with NACO at the ESO VLT. While many extra-solar planets and planetary candidates have been found in close orbits around stars by the radial velocity and the transit method, direct detections at wider orbits are rare. The Ursa Major (UMa) group, a young stellar association at an age of about 200-600 Myr and an average distance of 25 pc, has not yet been addressed as a whole although its members represent a very interesting sample to search for and characterize sub-stellar companions by direct imaging. Our goal was to find or to provide detection limits on wide sub-stellar companions around nearby UMa group members using high-resolution imaging. We searched for faint companions around 20 UMa group members within 30 pc. The primaries were placed below a semi-transparent coronagraph, a rather rarely used mode of NACO, to increase the dynamic range of the images. In most cases, second epoch images of companion candidates were taken to check whether they share common proper motion with the primary. Our coronagraphic images rule out sub-stellar companions around the stars of the sample. A dynamical range of typically 13-15 mag in the Ks band was achieved at separations beyond 3" from the star. Candidates as faint as Ks ~ 20 were securely identified and measured. The survey is most sensitive between separations of 100 and 200 au but only on average because of the very different target distance. Field coverage reaches about 650 au for the most distant targets. Most of the 200 candidates are visible in two epochs. All of those were rejected being distant background objects.

THE LEECH EXOPLANET IMAGING SURVEY: ORBIT AND COMPONENT MASSES OF THE INTERMEDIATE-AGE, LATE-TYPE BINARY NO UMa* †

ABSTRACT We present high-resolution Large Binocular Telescope LBTI/LMIRcam images of the spectroscopic and astrometric binary NO UMa obtained as part of the LBT Interferometer Exozodi Exoplanet Common Hunt exoplanet imaging survey. Our H-, K s -, and L′-band observations resolve the system at angular separations <0.″09. The components exhibit significant orbital motion over a span of ∼7 months. We combine our imaging data with archival images, published speckle interferometry measurements, and existing spectroscopic velocity data to solve the full orbital solution and estimate component masses. The masses of the K2.0 ± 0.5 primary and K6.5 ± 0.5 secondary are 0.83 ± 0.02 M ⊙ and 0.64 ± 0.02 M ⊙, respectively. We also derive a system distance of d = 25.87 ± 0.02 pc and revise the Galactic kinematics of NO UMa. Our revised Galactic kinematics confirm NO UMa as a nuclear member of the ∼500 Myr old Ursa Major moving group, and it is thus a mass and age benchmark. We compare the masses of the NO UMa binary components to those predicted by five sets of stellar evolution models at the age of the Ursa Major group. We find excellent agreement between our measured masses and model predictions with little systematic scatter between the models. NO UMa joins the short list of nearby, bright, late-type binaries having known ages and fully characterized orbits.

Spectroscopic signatures of youth in low-mass kinematic candidates of young moving groups

We present a study of age-related spectral signatures observed in 25 young low-mass objects that we have previously determined as possible kinematic members of five young moving groups: the Local Association (Pleiades moving group, age = 20–150 Myr), the Ursa Major group (Sirius supercluster, age = 300 Myr), the Hyades supercluster (age = 600 Myr), IC 2391 supercluster (age = 35–55 Myr) and the Castor moving group (age = 200 Myr). In this paper we characterize the spectral properties of observed high- or low-resolution spectra of our kinematic members by fitting theoretical spectral distributions. We study signatures of youth, such as lithium i 6708Å, Hα emission and other age-sensitive spectroscopic signatures in order to confirm the kinematic memberships through age constraints. We find that 21 (84 per cent) targets show spectroscopic signatures of youth in agreement with the age ranges of the moving group to which membership is implied. For two further objects, age-related constraints remain difficult to determine from our analysis. In addition, we confirm two moving group kinematic candidates as brown dwarfs.

The Hercules-Lyra association revisited

The Her-Lyr assoc., a nearby young MG, contains a few tens of ZAMS stars of SpT F to M. The existence and the properties of the Her-Lyr assoc. are controversial and discussed in the literature. The present work reassesses properties and the member list of Her-Lyr assoc., based on kinematics and age. Many objects form multiple systems or have low-mass companions and so we need to account for multiplicity. We use our own new imaging obs. and archival data to identify multiple systems. The colors and magnitudes of kinematic candidates are compared to isochrones. We derive further information on the age based on Li depletion, rotation, and coronal and chromospheric activity. A set of canonical members is identified to infer mean properties. Membership criteria are derived from the mean properties and used to discard non-members. The candidates selected from the literature belong to 35 stellar systems, 42.9% of which are multiple. Four multiple systems are confirmed in this work by common proper motion. An orbital solution is presented for the binary system HH Leo B and C. Indeed, a group of candidates displays signatures of youth. 7 canonical members are identified. The distribution of EWLi of canonical Her-Lyr members is spread widely and is similar to that of the Pleiades and the UMa group. Gyrochronology gives an age of 257+-46 Myr which is in between the ages of the Pleiades and the Ursa Major group. The measures of chromospheric and coronal activity support the young age. Four membership criteria are presented based on kinematics, EWLi, chromospheric activity, and gyro. age. In total, 11 stars are identified as certain members including co-moving objects plus additional 23 possible members while 14 candidates are doubtful or can be rejected. A comparison to the mass function, however, indicates the presence of a large number of additional unidentified low-mass members.

A spectroscopic study of DD UMa: Ursa Major group member and candidate for BRITE

The Ursa Major group is a nearby stellar supercluster which, while not gravitationally bound, is defined by co-moving members. DD UMa is a δ Scuti star whose membership in the Ursa Major group is unclear. The objective of this study is to confirm the membership of DD UMa in the Ursa Major group, as well as perform a detailed spectral analysis of the star. Since DD UMa is a low-amplitude δ Scuti star, we performed a frequency analysis. We determined fundamental parameters, chemical abundances, and derive a mass and age for the star. For this study we observed DD UMa at the Okayama Astrophysical Observatory with the high-resolution spectrograph HIDES, between the 27th of February and the 4th March, 2009. Additional observations were extracted from the ELODIE archive in order to expand our abundance analysis. Group membership of DD UMa was assessed by examining the velocity of the star in Galactic coordinates. Pulsational frequencies were determined by examining line profile variability in the HIDES spectra. Stellar fundamental parameters and chemical abundances were derived by fitting synthetic spectra to both the HIDES and ELODIE observations. DD UMa is found to be a member of the extended stream of the Ursa Major group, based on the space motion of the star. This is supported by the chemical abundances of the star being consistent with those of Ursa Major group members. The star is found to be chemically solar, with T eff = 7450 ± 150 K and log g = 3.98 ± 0.2. We found pulsational frequencies of 9.4 and 15.0 c/d. While these frequencies are insufficient to perform an asteroseismic study, DD UMa is a good bright star candidate for future study by the BRITE-constellation.

Spectroscopic properties of cool Ursa Major group members

CONTEXT: Most current radial velocity planet search programs have concentrated on stars of one solar mass. Our knowledge on the frequency of giant planets and brown dwarf companions to more massive stars is thus rather limited. In the case of solar-like stars, the frequency of short-period brown dwarf companions and very massive planets seems to be low. AIMS: Here we present evidence for a substellar companion to 30 Ari B, an F-star of 1.16 $\pm$ 0.04 $\rm M_\odot$ that is a member of a hierarchical triple system. METHODS: The companion was detected by means of precise radial velocity measurements using the 2-m Alfred-Jensch telescope and its 'echelle spectrograph. An iodine absorption cell provided the wavelength reference for precise stellar radial velocity measurements. RESULTS: We analyzed our radial velocity measurements and derived an orbit to the companion with period, P= 335.1+/-2.5 days, eccentricity e = 0.289+/-0.092, and mass function f(m) = (6.1+/-1.7)*10E-7 Modot. CONCLUSIONS: We conclude that the radial velocity variations of 30 Ari B are due to a companion with $m$ sin $i$ of $9.88\pm0.94$ $\rm M_{Jup}$ that is either a massive planet or a brown dwarf. {The object thus belongs to the rare class of massive planets and brown dwarfs orbiting main- sequence stars.

Neutrinos as galactic dark matter in the Ursa Major galaxy group?

Abstract We present the analysis of 23 published rotation curves of disc galaxies belonging to the Ursa Major group of galaxies, with kinematics free of irregularities. The rotation curves are analysed in the context of modified Newtonian dynamics (MOND). We add an extra component to the rotation curve fits, in addition to the stellar and gaseous discs: a speculative halo of constant density made of, for example, neutrinos, which would solve the bulk of the problem currently faced by MOND on rich galaxy cluster scales. We find that this additional unseen mass density is poorly constrained (as expected a priori, given that a neutrino halo never dominates the kinematics), but we also find that the best-fitting value is non-zero: ρ= 3.8 × 10−27 g cm−3, and that a zero density is marginally excluded with 87 per cent confidence; also, the 95 per cent confidence upper limit for the density is ρ= 9.6 × 10−27 g cm−3. These limits are slightly above the expectations from the Tremaine–Gunn phase-space constraints on ordinary 2 eV neutrinos, but in accordance with the maximum density expected for one or two species of 5 eV sterile neutrinos.

Spatial and kinematical lopsidedness of atomic hydrogen in the Ursa Major group of galaxies

We have carried out the harmonic analysis of the atomic hydrogen (HI) surface density maps and the velocity fields for 11 galaxies belonging to the Ursa Major group, over a radial range of 4-6 disc scalelengths in each galaxy. This analysis gives the radial variation of spatial lopsidedness, quantified by the Fourier amplitude A(1) of the m = 1 component normalized to the average value. The kinematical analysis gives a value for the elongation of the potential to be similar to 10 per cent. The mean amplitude of spatial lopsidedness is found to be similar to 0.14 in the inner disc, similar to the field galaxies, and is smaller by a factor of similar to 2 compared to the Eridanus group galaxies. It is also shown that the average value of A(1) does not increase with the Hubble type, contrary to what is seen in field galaxies. We argue that the physical origin of lopsidedness in the Ursa Major group of galaxies is tidal interactions, albeit weaker and less frequent than in Eridanus. Thus systematic studies of lopsidedness in groups of galaxies can provide dynamical clues regarding the interactions and evolution of galaxies in a group environment.

Unconstrained Astrometric Orbits for Hipparcos Stars with Stochastic Solutions

A considerable number of astrometric binaries whose positions on the sky do not obey the standard model of mean position, parallax, and linear proper motion were observed by the Hipparcos satellite. Some of them remain undiscovered, and their observational data have not been properly processed with the more adequate astrometric model that includes nonlinear orbital motion. We develop an automated algorithm, based on "genetic optimization," to solve the orbital fitting problem in the most difficult setup, when no prior information about the orbital elements is available (from, e.g., spectroscopic data or radial velocity monitoring). We also offer a technique to accurately compute the probability that an orbital fit is bogus, that is, that an orbital solution is obtained for a single star, and to estimate the probability distributions for the fitting orbital parameters. We test this method on Hipparcos stars with known orbital solutions in the catalog and further apply it to 1561 stars with stochastic solutions, which may be unresolved binaries. At a confidence level of 99%, orbital fits are obtained for 65 stars, most of which have not been known as binary. It is found that reliable astrometric fits can be obtained even if the period is somewhat longer than the time span of the Hipparcos mission, that is, if the orbit is not closed. A few of the new probable binaries with A-type primaries with periods 444-2015 days are chemically peculiar stars, including Ap and λ Bootis types. The anomalous spectra of these stars are explained by admixtures of light from the unresolved, sufficiently bright and massive companions. We estimate the apparent orbits of four stars that have been identified as members of the ≈300 Myr old Ursa Major kinematic group. Another four new nearby binaries may include low-mass M-type or brown dwarf companions. Follow-up spectroscopic observations in conjunction with more accurate inclination estimates will lead to better estimates of the secondary mass. Similar astrometric models and algorithms can be used for binary stars and planet hosts observed by SIM and Gaia.

Abundances of a sample of A and F-type dwarf members of the Ursa Major Group

Abundances of 11 chemical elements have been determined for 10 F and 12 A dwarfs (“normal” and chemically peculiar) bona-fide and probable members of the Ursa Major Group (age about 500 Myr). The abundances were determined in a uniform manner using a model atmosphere analysis by minimising the chi-square of grids of synthetic spectra to observed high resolution high S/ N( R � 25 000 and R � 70 000) spectra obtained in three narrow spectral regions centered around 5075 A, 5525 A and 6160 A. Specifically, Takeda’s (1995) semiautomated procedure was used to derive the abundances of C, O, Na, Mg, Si, Ca, Sc, Fe, Ni, Y, and Ba, the projected rotational velocity ve sin i and the microturbulent velocity for each star analysed. In graphs [X/H] versus Teff, the A stars exhibit larger star-to-star variations in [Fe/H], [Ni/H], and [Si/H] than the F dwarfs do. The abundance of nickel is the only one that appears to be correlated with that of iron.

Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group

Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members, based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for � 15% of objects, perhaps considerably less. Our UMa members demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent studies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our final UMa group members indicates an age of 500 � 100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members’ mean chromospheric emission levels, rotational velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200–500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission, but several may show residual fluxes a factor of � 2 below a Hyades-defined lower envelope. If one defines a Maunder-like minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity, which is consistent with estimates from older field stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars defining intermediate-age star streams in previous studies, identify a small number of potentially very young but isolated field stars, note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main sequence, unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs, and argue that some extant transformations of activity indices are not adequate for cool dwarfs, for which Ca ii infrared triplet emission seems to be a better proxy than H� -based values for Ca ii H and K indices.

Stellar atmospheres of nearby young solar analogs

High-resolution ( R≈90,000) spectra of 34 nearby, young Sun-like stars were analyzed using stellar atmosphere models to estimate effective photosphere temperatures, surface gravities, and the abundance of certain heavy elements (C, Na, Mg, Si, S, Ca, Ti, Fe, and Ni). The effective temperatures derived from spectroscopy were compared with temperatures estimated using optical and near-infrared photometry. In many cases the spectroscopic temperatures are significantly higher than the photometric estimates, possibly as a result of spottedness or chromospheric activity on these active stars. Values of effective temperature, surface gravity, and luminosity were compared to theoretical stellar evolution tracks and the evolutionary status of these objects was evaluated. The correlation between heavy element abundance patterns and kinematics (space motion) was also examined. Two nearby stars that were tentatively assigned to the Hyades cluster based on kinematics have Fe abundances that are also consistent with membership in that cluster. Members of the Ursa Major kinematic group exhibit a range of [Fe/H] values but have monotonic [Si/Fe]. These two observations suggest that heterogeneous incorporation of the heavy elements into protostars is creating the variation in metallicity. Local Association members have a distinctly different Si/Fe that probably reflects their distinct origin and chemical inheritance.

Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups

We present here high resolution echelle spectra taken during three observing runs of 14 single late-type stars identified in our previous studies (Montes et al. 2001b, hereafter Paper I) as possible members of different young stellar kinematic groups (Local Association (20 - 150 Myr), Ursa Major group (300 Myr), Hyades supercluster (600 Myr), and IC 2391 supercluster (35 Myr)). Radial velocities have been determined by cross correlation with radial velocity standard stars and used together with precise measurements of proper motions and parallaxes taken from Hipparcos and Tycho-2 Catalogues, to calculate Galactic space motions (U, V, W) and to apply Eggen's kinematic criteria. The chromospheric activity level of these stars have been analysed using the information provided for several optical spectroscopic features (from the Ca II H & K to Ca II IRT lines) that are formed at different heights in the chromosphere. The Li I 6707.8 AA line equivalent width (EW) has been determined and compared in the EW(Li I) versus spectral type diagram with the EW(Li I) of stars members of well known young open clusters of different ages, in order to obtain an age estimation. All these data allow us to analyse in more detail the membership of these stars in the different young stellar kinematic groups. Using both kinematic and spectroscopic criteria we have confirmed PW And, V368 Cep, V383 Lac, EP Eri, DX Leo, HD 77407, and EK Dra as members of the Local Association and V834 Tau, pi^{1} UMa, and GJ 503.2 as members of the Ursa Major group. A clear rotation-activity dependence has been found in these stars.

Late-type members of young stellar kinematic groups - I. Single stars

This is the first paper of a series aimed at studying the properties of late-type members of young stellar kinematic groups. We concentrate our study on classical young moving groups such as the Local Association (Pleiades moving group, 20-150 Myr), IC 2391 supercluster (35 Myr), Ursa Major group (Sirius supercluster, 300 Myr), and Hyades supercluster (600 Myr), as well as on recently identified groups such as the Castor moving group (200 Myr). In this paper we compile a preliminary list of single late-type possible members of some of these young stellar kinematic groups. Stars are selected from previously established members of stellar kinematic groups based on photometric and kinematic properties as well as from candidates based on other criteria such as their level of chromospheric activity, rotation rate and lithium abundance. Precise measurements of proper motions and parallaxes taken from the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, and published radial velocity measurements are used to calculate the Galactic space motions (U, V W) and to apply Eggen's kinematic criteria in order to determine the membership of the selected stars to the different groups. Additional criteria using age-dating methods for late-type stars will be applied in forthcoming papers of this series. A further study of the list of stars compiled here could lead to a better understanding of the chromospheric activity and their age evolution, as well as of the star formation history in the solar neighbourhood. In addition, these stars are also potential search targets for direct imaging detection of substellar companions.

The geometry of stellar motions in the nucleus region of the Ursa Major kinematic group

The structure of the central part of the UMa stellar kinematic group was considered according to the data from Hipparcos, Tycho and ACT catalogues. We have shown that in the space occupied by the nucleus there are several populations of stars. The first includes ten stars which are very compactly located in the proper motion diagram and have a normal proper motion distribution. The second group includes six stars with large kinematic difference and there are six field stars placed here too. The question about their coexistence in a common region of space remains open. The well-known stars Mizar and Alcor appear to belong to the different subsystems and do not belong to a unique stellar system. The published radial velocities, as a rule, are determined with insufficient accuracy compared to the errors of the currently available proper motions. A high precision in the radial velocities measurements is necessary for a thorough study of the kinematics of stars inside UMa star system.

HR 6094: A Young Solar-Type, Solar-Metallicity Barium Dwarf Star

The young solar-type star HR 6094 is found to be a barium dwarf, overabundant in the s-process elements as well as deficient in C. It is a member of the solar-metallicity, 0.3 Gyr old Ursa Major kinematical group. Measurements of radial velocity and ultraviolet flux do not support the attribution of such abundance anomalies to an unseen degenerate companion. A common proper motion, V = 10, DA white dwarf (WD), located 5360 AU away, however, strongly supports the explanation of the origin of this barium star by the process of mass transfer in a binary system, in which the secondary component accreted matter from the primary one (now the WD) when it was an asymptotic giant branch (AGB) star self-enriched in the s-process elements. The membership in the UMa group of another s-process-rich and C-deficient star, HR 2047, suggests that these stars could have formed a multiple system in the past, which was disrupted by the mass-loss episode of the former AGB star. Their [C/Fe] deficiency could be explained by the action of the hot-bottomed envelope burning process in the late AGB, thereby reconverting it from a C-rich to an O-rich star, depleting C while enriching its envelope with Li and neutron capture elements. This is the first identification of the barium phenomenon in a near-zero-age star, besides being the first barium system in which the remnant of the late AGB star responsible for the heavy-element enrichment may have been directly spotted.

Lithium Abundances of Solar-Type Stars in Open Clusters

This paper discusses the lithium abundances of late-F through mid-F dwarfs in the nearby open clusters. That means that I am considering young disk stars, of course, with metallicities similar to the Sun’s. I will not discuss the chasm seen among mid-F dwarfs. Li is not easily studied because only one spectroscopic feature is available and that feature is often weak. Thus high spectroscopic resolution and good signal to noise are needed. But Li is still the most easily studied of the light-element pentateuch (D, He, Li, Be, B).The few clusters now accessible to large telescopes at high spectroscopic resolution include the Hyades, Pleiades, a Persei, as well as smaller samples in a few others (such as Coma, M67, NGC 752, the Ursa Major Group). The samples within clusters are often substantial, and allow one to examine differences in Li between stars of the same mass, age, and composition. However the total number of clusters observed is still small, so one must assume that each cluster is fully representative for its age and composition.Much recent work on Li in stars has been for Population II objects because of the importance of Li for cosmology. We study Li in Pop I stars for three reasons: First, to understand how the Galaxy has come to be enriched in Li since the Pop II stars formed. Second, to understand the stellar physics behind Li depletion in Pop I stars as a means to understanding what happens in the outer layers of such a star over time. Third, to apply that understanding of the stellar physics to Pop II stars so we can correctly infer the Big Bang Li abundance.

Oxygen abundances in F-type stars of the Hyades and the Ursa Major group

We have derived the oxygen abundances of 50 F-type main-sequence stars, belonging to the Hyades open cluster, the Ursa Major group, and the field, using an NLTE analysis of the infrared triplet lines of O I at λλ7771-7775. The stars of the middle F-range (6500<T eff <6900 K) have previously been shown to exhibit a marked reduction in lithium abundance compared with both cooler and hotter objects in the same systems: the Li gap, for which several explanations have been proposed. Our results here show a substantial measure of uniformity in the oxygen abundances over the range in T eff between 5800 and 7400 K. The data might indicate a small dip (<0.1 dex) in the oxygen abundance for stars located in the Li gap