Optical Absolute Magnitude Research Articles

Why Are Some Radio Galaxies Detected by Fermi, but Others Not?

Aiming to answer an interesting question on why some radio sources can be detected by Fermi-LAT but others cannot, we compare several parameters of Fermi-detected Fanaroff-Riley radio galaxies (FFRs) and non-Fermi-detected sources (NFFRs), including the optical absolute magnitude, 1.4 GHz radio luminosity, radio loudness, core dominance parameter, Doppler factor, and the mass of the central black hole. Significant differences are ascertained within these parameters between FFRs and NFFRs. Our findings suggest that FFRs are jet-dominant while NFFRs are disk-dominant sources, and NFFRs have a weaker beaming effect. Additionally, we predict the observed γ-ray flux for NFFRs, establishing that the reason why some NFFRs are not detectable arises from their γ-ray flux being below the sensitivity detection threshold of Fermi. We also discuss two sub-types of Fanaroff-Riley galaxies, namely FR I and FR II sources. We first propose a “changing-look” phenomenon in these radio sources and also investigate why FR IIs seem to be exclusive in γ-ray emission.

Effects of Winds on the Optical Properties of Type Ib and Ic Supernova Progenitors

Abstract We investigate the effects of winds on the observational properties of Type Ib and Ic supernova (SN Ib/Ic) progenitors using spectral models constructed with the non-LTE stellar atmospheric code CMFGEN. We consider SN Ib/Ic progenitor models of the final mass range of 2.16–9.09 M ⊙ having different surface temperatures and chemical compositions, and calculate the resulting spectra for various wind mass-loss rates and wind terminal velocities. We find that the progenitors having an optically thick wind would become brighter in the optical for a higher mass-loss rate (or a lower wind terminal velocity) because of the formation of the photosphere in the extended wind matter and the contribution from free–free and line emissions from the wind. As a result, for the standard Wolf-Rayet wind mass-loss rate, helium-deficient compact SN Ic progenitors would be brighter in the optical by ∼3 mag compared to the case without the wind effects. We also find that the color dependence on the photospheric temperature is non-monotonic because of the wind effects. Our results imply that inferring the progenitor mass, bolometric luminosity, and effective temperature from the optical observation using the standard stellar evolution model prediction can be misleading. By comparing our fiducial model predictions with the detection limits of the previous SN Ib/Ic progenitor searches, we conclude that a deep search with an optical absolute magnitude larger than ∼−4 is needed to directly identify most of the ordinary SN Ib/Ic progenitors. We discuss implications of our results for the observed SN Ib/Ic progenitor candidates for iPTF13bvn, SN 2019vyr, and SN 2017ein.

Discovery of a new nearby globular cluster with extreme kinematics located in the extension of a halo stream

Context. We report the discovery of VVV-CL160, a new nearby globular cluster (GC) with extreme kinematics, located in the Galactic plane at l = 10.1477 deg, b = 0.2999 deg. Aims. We aim to characterize the physical properties of this new GC and place it in the context of the Milky Way, exploring its possible connection with the known GC NGC 6544 and with the Hrid halo stream. Methods. VVV-CL160 was originally detected in the VISTA Variables in the Vía Láctea (VVV) survey. We use the proper motions (PMs) from the updated VVV Infrared Astrometric Catalog (VIRAC2) to select GC members and make deep near-infrared color-magnitude diagrams to study the cluster properties. We also fit King models to the decontaminated sample to determine the GC structural parameters. Results. VVV-CL160 has an unusually large PM for a Galactic GC as measured with VIRAC2 and Gaia EDR3: μα cos (δ) = −2.3 ± 0.1 mas yr−1 and μδ = −16.8 ± 0.1 mas yr−1. The kinematics are similar to those of the known GC NGC 6544 and the Hrid halo stream. We estimate a reddening of E(J − K) = 1.95 mag and an extinction of AK = 1.40 mag for VVV-CL160. We also measure a distance modulus of (m − M) = 13.01 mag and a distance of D⊙ = 4.0 ± 0.5 kpc. This places the GC at z = 29 pc above the Galactic plane and at a galactocentric distance of RG = 4.2 kpc. We also measure a metallicity of [Fe/H] = −1.4 ± 0.2 dex for an adopted age of t = 12 Gyr; King model fits of the PM-decontaminated sample reveal a concentrated GC, with core radius rc = 22.8″ and tidal radius rt = 50′. We also estimate the absolute magnitude in the near-infrared of MK = −7.6 mag, equivalent to an optical absolute magnitude of MV = −5.1 mag. We also explore the possible association of this new GC with other GCs and halo streams. Conclusions. Based on the locations and kinematics, we suggest that VVV-CL160, along with NGC 6544, may be associated with the extension of the Hrid halo stream.

Studying the Flux Density of Bright Active Galaxies at Different Spectral Bands

Statistical studies are reported in this article for an active galactic nuclei sample of different type of active galaxies Seyferts 1, Seyferts 2, and Quasars. These sources have been selected from a Catalogue for bright X-ray galaxies. The name of this index is ROSAT Bright Source Catalogue (RBSC) and the NRAO VLA Sky Survey (NVSS). In this research, multi-wavelength observational bands Radio at 1.4 GHz, Optical at 4400 A0, and X-ray at energy 0.1-2.4 KeV have been adopted in this study. The behavior of flux density ratios has been studied , with respect to the absolute magnitude . Furthermore, the Seyfert1 and Seyfert 2 objects are combined in one group and the QSOs are collectest in another group. Also, it has been found that the ratios , are increasing towards fainter optical absolute magnitude especially in Quasars.

Studying the Flux Density of Bright Active Galaxies at Different Spectral Bands

Statistical studies are reported in this article for an active galactic nuclei sample of different type of active galaxies Seyferts 1, Seyferts 2, and Quasars. These sources have been selected from a Catalogue for bright X-ray galaxies. The name of this index is ROSAT Bright Source Catalogue (RBSC) and the NRAO VLA Sky Survey (NVSS). In this research, multi-wavelength observational bands Radio at 1.4 GHz, Optical at 4400 A0, and X-ray at energy 0.1-2.4 KeV have been adopted in this study. The behavior of flux density ratios has been studied , with respect to the absolute magnitude . Furthermore, the Seyfert1 and Seyfert 2 objects are combined in one group and the QSOs are collectest in another group. Also, it has been found that the ratios , are increasing towards fainter optical absolute magnitude especially in Quasars.

Multiwavelength approach to classifying transient events in the direction of M 31

Context. In the hunt for rare time-domain events, it is important to consider confusing exotic extragalactic phenomena with more common Galactic foreground events. Aims. We show how observations from multiple wavebands, in this case optical and X-ray observations, can be used to facilitate the distinction between the two. Methods. We discovered an extremely bright and rapid transient event during optical observations of the M 31 galaxy taken by the intermediate Palomar Transient Factory (iPTF). The persistent optical counterpart of this transient was previously thought to be a variable star in M 31 without any dramatic flux excursions. The iPTF event initially appeared to be an extraordinarily rapid and energetic extragalactic transient, which had a ≈3 mag positive flux excursion in less than a kilosecond; one of the exciting possibilities was this event could be a very fast nova in M 31. The nature of the source was resolved with the help of Chandra archival data, where we found an X-ray counterpart and obtained its X-ray spectrum. Results. We find the X-ray spectrum of the quiescent emission can be described by a model of optically thin plasma emission with a temperature of ≈7 MK, typical for coronal emission from an active star. The combination of the X-ray luminosity, which is calculated assuming the source is located in M 31 (~3 × 1036 erg s−1), and the color temperature exclude any type of known accreting compact object or active star in M 31. We argue instead that the optical transient source is an M-type main-sequence, active star located in the disk of the Milky Way at a distance of ~0.5–1 kpc. Its persistent X-ray luminosity is in the ≈1.3–5 × 1030 erg s−1 range and it has the absolute optical magnitude of 9.5–11.0 mag in the R band. The observed optical flare has the equivalent duration of ≈95 min and total energy of ≈(0.3–1) × 1035 erg in the R band, which places it among the brightest flares ever observed from an M-type star. This case can serve as an example for the classification of Galactic and extragalactic events in upcoming high-cadence time-domain projects, such as the Zwicky Transient Facility and the Large Synoptic Survey Telescope.

(Almost) Dark Galaxies in the ALFALFA Survey: Isolated H i-bearing Ultra-diffuse Galaxies

Abstract We present a sample of 115 very low optical surface brightness, highly extended, H i-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered “ultra-diffuse” galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H i. We find that while these sources have normal star formation rates for H i-selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H i-synthesis follow-up imaging of three of these H i-bearing ultra-diffuse sources. We measure H i diameters extending to ∼40 kpc, but note that while all three sources have large H i diameters for their stellar mass, they are consistent with the H i mass–H i radius relation. We further analyze the H i velocity widths and rotation velocities for the unresolved and resolved sources, respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H i-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.

Searching for metal-deficient emission-line galaxy candidates: the final sample of the SDSS DR12 galaxies

We present a spectroscopic study of metal-deficient dwarf galaxy candidates, selected from the SDSS DR12. The oxygen abundances were derived using the direct method in galaxies with the electron temperature-sensitive emission line [OIII]4363A measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [OIII]4363A line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 low-metallicity candidates with oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient (XMD) candidates with 12+log O/H<7.35. Ten out of sixteen XMDs known so far (or ~60%) have been discovered by our team using the direct method. Three XMDs were found in the present study. We study relations between global parameters of low-metallicity galaxies, including absolute optical magnitudes, Hbeta luminosities (or equivalently star formation rates), stellar masses, mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies strongly deviate to lower metallicities in L-Z, L(Hbeta)-Z and Mstar-Z diagrams than in relations obtained for large samples of low-redshift, star-forming galaxies with non-restricted metallicities. These less chemically evolved galaxies with stellar masses ~10^6-10^8Msun, Hbeta luminosities ~10^38-10^41 erg/s, SFR~0.01-1.0Msun/yr, and sSFR~50 Gyr^-1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.

On the universality of luminosity–metallicity and mass–metallicity relations for compact star-forming galaxies at redshifts 0 &lt; z &lt; 3

We study relations between global characteristics of low-redshift (0 < z < 1) compact star-forming galaxies, including absolute optical magnitudes, H β emission-line luminosities (or equivalently star formation rates), stellar masses, and oxygen abundances. The sample consists of 5182 galaxies with high-excitation H ii regions selected from the SDSS DR7 and SDSS/BOSS DR10 surveys adopting a criterion [O iii] λ4959/H β ≥ 1. These data were combined with the corresponding data for high-redshift (2 ≲ z ≲ 3) star-forming galaxies. We find that in all diagrams low-z and high-z star-forming galaxies are closely related indicating a very weak dependence of metallicity on stellar mass, redshift, and star formation rate. This finding argues in favour of the universal character of the global relations for compact star-forming galaxies with high-excitation H ii regions over redshifts 0 < z < 3.

THE FIELD X-RAY AGN FRACTION TOz= 0.7 FROM THECHANDRAMULTIWAVELENGTH PROJECT AND THE SLOAN DIGITAL SKY SURVEY

We employ the Chandra Multiwavelength Project (ChaMP) and the Sloan Digital Sky Survey (SDSS) to study the fraction of X-ray-active galaxies in the field out to z = 0.7. We utilize spectroscopic redshifts from SDSS and ChaMP, as well as photometric redshifts from several SDSS catalogs, to compile a parent sample of more than 100,000 SDSS galaxies and nearly 1,600 Chandra X-ray detections. Detailed ChaMP volume completeness maps allow us to investigate the local fraction of active galactic nuclei (AGN), defined as those objects having broad-band X-ray luminosities L_X (0.5-8 keV) > 10^42 erg s^-1, as a function of absolute optical magnitude, X-ray luminosity, redshift, mass, and host color/morphological type. In five independent samples complete in redshift and i-band absolute magnitude, we determine the field AGN fraction to be between 0.16 +/- 0.06% (for z < 0.125 and -18 > M_i > -20) and 3.80 +/- 0.92% (for z < 0.7 and M_i < -23). We find striking agreement between our ChaMP/SDSS field AGN fraction and the Chandra cluster AGN fraction, for samples restricted to similar redshift and absolute magnitude ranges: 1.19 +/- 0.11% of ChaMP/SDSS field galaxies with 0.05 < z < 0.31 and absolute R-band magnitude more luminous than M_R < -20 are AGN. Our results are also broadly consistent with measures of the field AGN fraction in narrow, deep fields, though differences in the optical selection criteria, redshift coverage, and possible cosmic variance between fields introduce larger uncertainties in these comparisons.

EXTENDED RADIO EMISSION IN MOJAVE BLAZARS: CHALLENGES TO UNIFICATION

We present the results of a study on the 1.4 GHz kpc-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud AGNs. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. Many BL Lacs exhibit extended radio power and kpc-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs. This poses challenges to the simple radio-loud unified scheme, which links BL Lacs to FRIs and quasars to FRIIs. We find a significant correlation between extended radio emission and pc-scale jet speeds: the more radio powerful sources possess faster jets. This indicates that the 1.4 GHz (or low frequency) radio emission is indeed related to jet kinetic power. Various properties such as extended radio power and apparent pc-scale jet speeds vary smoothly between different blazar subclasses, suggesting that, at least in terms of radio jet properties, the distinction between quasars and BL Lac objects, at an emission-line equivalent width of 5 Angstrom is essentially an arbitrary one. Based on the assumption that the extended radio luminosity is affected by the kpc-scale environment, we define the ratio of extended radio power to absolute optical magnitude as a proxy for environmental effects. Trends with this parameter suggest that the pc-scale jet speeds and the pc-to-kpc jet misalignments are not affected by the large-scale environment, but are more likely to depend upon factors intrinsic to the AGN, or its local pc-scale environment. We suggest that some of the extremely misaligned MOJAVE blazar jets could be "hybrid" morphology sources, with an FRI jet on one side and an FRII jet on the other. (Abridged)

Near-Eddington Luminosity Flares from Quiescent Supermassive Black Holes

AbstractA dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes within its tidal disruption radius, is disrupted, and a flare of electromagnetic radiation is emitted when the bound stellar debris is accreted. Although the tidal disruption of a star is a rare event in a galaxy, ~ 10−4 yr−1, observational candidates have emerged in all-sky X-ray and deep UV surveys in the form of luminous UV/X-ray flares from otherwise quiescent galaxies. We present the light curves and broadband properties of three tidal disruption candidates discovered in the UV by GALEX, and find that (1) the light curves are well-fitted by the power-law decline expected for the fallback of debris from a tidally disrupted solar-type star, and (2) the UV/optical spectral energy distributions can be attributed to thermal emission from an envelope of debris located at ten times the tidal disruption radius of the central black hole. We use the observed peak absolute optical magnitudes of the flares to predict the detection capabilities of the next generation of wide-field optical synoptic surveys.

LUMINOUS THERMAL FLARES FROM QUIESCENT SUPERMASSIVE BLACK HOLES

A dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces, and a flare of electromagnetic radiation is emitted when the bound fraction of the stellar debris falls back onto the black hole and is accreted. Here we present the third candidate tidal disruption event discovered in the GALEX Deep Imaging Survey: a 1.6x10^{43} erg s^{-1} UV/optical flare from a star-forming galaxy at z=0.1855. The UV/optical SED during the peak of the flare measured by GALEX and Palomar LFC imaging can be modeled as a single temperature blackbody with T_{bb}=1.7x10^{5} K and a bolometric luminosity of 3x10^{45} erg s^{-1}, assuming an internal extinction with E(B-V)_{gas}=0.3. The Chandra upper limit on the X-ray luminosity during the peak of the flare, L_{X}(2-10 keV)< 10^{41} erg s^{-1}, is 2 orders of magnitude fainter than expected from the ratios of UV to X-ray flux density observed in active galaxies. We compare the light curves and broadband properties of all three tidal disruption candidates discovered by GALEX, and find that (1) the light curves are well fitted by the power-law decline expected for the fallback of debris from a tidally disrupted solar-type star, and (2) the UV/optical SEDs can be attributed to thermal emission from an envelope of debris located at roughly 10 times the tidal disruption radius of a ~10^{7} M_sun central black hole. We use the observed peak absolute optical magnitudes of the flares (-17.5 > M_{g} > -18.9) to predict the detection capabilities of upcoming optical synoptic surveys. (Abridged)

ON THE BL LACERTAE OBJECTS/RADIO QUASARS AND THE FR I/II DICHOTOMY

In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow-Owen dividing line for FR I/FR II dichotomy in the optical absolute magnitude of host galaxy-radio luminosity (M_R-L_Rad) plane can be translated to the line in the black hole mass-jet power (M_bh-Q_jet) plane by using two empirical relations: Q_jet-L_Rad and M_bh}-M_R. We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow & Owen FR I/II dividing line in the M_bh-Q_jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L_bol/L_Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L_bol/L_Edd 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.

Limits on the X-ray and optical luminosity of the progenitor of the Type Ia supernova 2007sr

We present Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC2), Galaxy Evolution Explorer (GALEX) and Chandra observations of the position of the Type la supernova 2007sr in the Antennae galaxy, taken before the explosion. No source is found in any of the observations, allowing us to put interesting constraints on the progenitor luminosity. In total there is about 450 ks of Chandra data, spread over seven different observations. Limiting magnitudes of far-ultraviolet (FUV) (23.7 AB mag), near-ultraviolet (NUV) (23.8 AB mag), F555W (26.5 Vega mag) and F814W (24.5-25 Vega mag) are derived. The distance to the Antennae galaxy is surprisingly poorly known, with almost a factor of 2 difference between the latest distance based on the tip of the red giant branch (13.3 Mpc) and the distance derived from the 2007sr light curve (25 Mpc). Using these distances we derive limits on absolute optical and UV magnitudes of any progenitor but these are still above the brightest (symbiotic) proposed progenitors. From the Chandra data a 3σ upper limit to the X-ray luminosity of 0.5-8.0 x 10 37 erg s -1 in the 0.3-1 keV range is found. This is below the X-ray luminosity of the potential progenitor of the Type Ιa supernova 2007on that we recently discovered and for which we report a corrected X-ray luminosity. If that progenitor is confirmed it suggests the two supernovae have different progenitors. The X-ray limit is comparable to the brightest supersoft X-ray sources in the Galaxy, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) and significantly below the luminosities of the brightest supersoft and quasi-soft X-ray sources found in nearby galaxies, ruling out such sources as progenitors of this Type Ia supernova.

The nature of the Sloan Digital Sky Survey galaxies in various classes based on morphology, colour and spectral features - II. Multi-wavelength properties

We present a multi-wavelength study of the nature of the SDSS galaxies divided into fine classes based on their morphology, colour and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert and LINER, which returns a total of 16 fine classes of galaxies. The properties of galaxies in each fine class are investigated from radio to X-ray, using 2MASS, IRAS, FIRST, NVSS, GALEX and ROSAT data. The UV - optical - NIR colours of blue early-type galaxies (BEGs) seem to result from the combination of old stellar population and recent star formation (SF). Non-passive red early-type galaxies (REGs) have larger metallicity and younger age than passive REGs, which implies that non-passive REGs have suffered recent SF adding young and metal-rich stars to them. The radio detection fraction of REGs strongly depends on their optical absolute magnitudes, while that of most late-type galaxies does not, implying the difference in their radio sources: AGN and SF. The UV - optical colours and the radio detection fraction of passive RLGs show that they have properties similar to REGs rather than non-passive RLGs. Dust extinction may not be a dominant factor making RLGs red, because RLGs are detected in the mid- and far-infrared bands less efficiently than blue late-type galaxies (BLGs). The passive BLGs have very blue UV - optical - NIR colours, implying either recent SF quenching or current SF in their outskirts. Including star formation rate, other multi-wavelength properties in each fine class are investigated, and their implication on the identity of each fine class is discussed (abridged).

The luminosities and distance scales of type II Cepheid and RR Lyrae variables

Infrared and optical absolute magnitudes are derived for the type II Cepheids κ Pav and VY Pyx using revised Hipparcos parallaxes and for κ Pav, V553 Cen and SW Tau from pulsational parallaxes. Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorily and are combined in deriving absolute magnitudes. Phase-corrected J, H and Ks mags are given for 142 Hipparcos RR Lyraes based on Two-Micron All-Sky Survey observations. Pulsation and trigonometrical parallaxes for classical Cepheids are compared to establish the best value for the projection factor (p) used in pulsational analyses. The MV of RR Lyrae itself is 0.16 ± 0.12 mag brighter than predicted from an MV–[Fe/H] relation based on RR Lyrae stars in the Large Magellanic Cloud (LMC) at a modulus of 18.39 ± 0.05 as found from classical Cepheids. This is consistent with the prediction of Catelan & Cortés that it is overluminous for its metallicity. The results for the metal- and carbon-rich Galactic disc stars, V553 Cen and SW Tau, each with small internal errors (±0.08 mag) have a mean deviation of only 0.02 mag from the period–luminosity (PL) relation established by Matsunaga et al. for type II Cepheids in globular clusters and with a zero-point based on the same LMC-scale. Comparing directly the luminosities of these two stars with published data on type II Cepheids in the LMC and in the Galactic bulge leads to an LMC modulus of 18.37 ± 0.09 and a distance to the Galactic Centre of R0= 7.64 ± 0.21 kpc. The data for VY Pyx agree with these results within the uncertainties set by its parallax. Evidence is presented that κ Pav may have a close companion and possible implications of this are discussed. If the pulsational parallax of this star is incorporated in the analyses, the distance scales just discussed will be increased by ∼0.15 ± 0.15 mag. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters. This is probably due to a narrower range of masses in the latter case.

Accurate absolute magnitudes for Kuiper belt objects and Centaurs

Accurate absolute optical magnitude values ( H V and H R ) for Kuiper belt objects (KBOs) and Centaurs are becoming increasingly important as observations in other wavelengths, particularly SIRTF thermal infrared measurements, become available for large samples of objects. We present accurate H V and H R values for 90 KBOs and Centaurs, based on our published optical photometry. We find that our H V values are in good agreement with those available from the European photometric survey of minor bodies in the outer Solar System. Comparison with H V values from the JPL Horizons database and the Minor Planet Center database shows that these sources are systematically brighter than ours by about 0.3 mag.

The spectroscopic characteristics of intermediate-aged pre-main-sequence stars: theηChamaeleontis cluster

We present a study of calibrated low-resolution spectra of the 18 known primaries of the ≈9-Myr-old η Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravity-sensitive indices derived from the spectra, we compare the η Cha stars to standard dwarfs. We find that the VRI colours of the PMS stars are indistinguishable from those of main-sequence stars, but that a B-band excess attaining ≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for the η Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung–Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-type η Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that the η Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirm AV= 0.4 mag likely due to the presence of circumstellar material.

Is there a dichotomy in the radio loudness distribution of quasars?

We present a new approach to tackling the issue of radio loudness in quasars. We constrain a (simple) prescription for the intrinsic distribution of radio-to-optical ratios by comparing properties of Monte Carlo simulated samples with those of observed optically selected quasars. We find strong evidence for a dependence of the radio luminosity on the optical one, even though with a large scatter. The dependence of the fraction of radio-loud quasars on apparent and absolute optical magnitudes results in a selection effect related to the radio and optical limits of current surveys. The intrinsic distribution of the radio-to-optical ratios shows a peak at R*1.4∼ 0.3, with only ≲5 per cent of objects being included in a high-R*1.4 tail, which identifies the radio-loud regime. No lack or deficit of sources — but only a steep transition region — is present between the radio-loud and radio-quiet populations at any R*1.4. Briefly, we discuss possible origins for this behaviour (e.g. an absence of jets in radio-quiet sources, a large range of radiative radio efficiency, different lifetimes for the accretion and jet ejection phenomena, etc.).