Radio-quiet Sources Research Articles

Upper Limits on the Radio Pulses from Magnetars and a Central Compact Object with FAST

Magnetars and central compact objects (CCOs) are subgroups of neutron stars that show a number of properties distinguished from canonical radio pulsars. We performed radio observations of three magnetars, SGR 0418+5729, 1E 2259+586, and 4U 0142+61, and a CCO, PSR J1852+0040, with the Five-hundred-meter Aperture Spherical radio Telescope at 1.25 GHz, aiming to search for radio pulsations in their quiescent states. During two observation epochs, no radio pulses have been detected toward any target above a significance of signal-to-noise ratio = 7 from either the direct folding or blind search. We provided the most stringent upper limit of radio flux (≲2–4 μJy) for the magnetars and the CCO. For the magnetars with long periods, the real upper limits are likely 1 order of magnitude larger due to the red noise. The deep radio observations suggest that these magnetars and the CCO are indeed radio-quiet sources or unfavorably beamed.

Exploring the radio loudness of SDSS quasars with spectral stacking

ABSTRACT We use new 144 MHz observations over 5634 deg2 from the LOFAR (Low Frequency Array) Two-metre Sky Survey (LoTSS) to compile the largest sample of uniformly selected, spectroscopically confirmed quasars from the 14th data release of the Sloan Digital Sky Survey (SDSS-DR14). Using the classical definition of radio loudness, R = log (L1.4GHz/Li), we identify 3697 radio-loud (RL) and 111 132 radio-quiet (RQ) sources at 0.6 < z < 3.4. To study their properties, we develop a new rest-frame spectral stacking algorithm, designed with forthcoming massively multiplexed spectroscopic surveys in mind, and use it to create high signal-to-noise composite spectra of each class, matched in redshift and absolute i-band magnitude. We show that RL quasars have redder continuum and enhanced [O ii] emission than their RQ counterparts. These results persist when additionally matching in black hole mass, suggesting that this parameter is not the defining factor in making a quasi-stellar object (QSO) RL. We find that these features are not gradually varying as a function of radio loudness, but are maintained even when probing deeper into the RQ population, indicating that a clear-cut division in radio loudness is not apparent. Upon examining the star formation rates (SFRs) inferred from the [O ii] emission line, with the contribution from active galactic nucleus removed using the [Ne v] line, we find that RL quasars have a significant excess of star formation relative to RQ quasars out to z = 1.9 at least. Given our findings, we suggest that RL sources either preferably reside in gas-rich systems with rapidly spinning black holes, or represent an earlier obscured phase of QSO evolution.

The Fundamental Planes of black hole activity for radio-loud and radio-quiet quasars

ABSTRACT We examine the Fundamental Plane of black hole activity for correlations with redshift and radio loudness in both radio-loud and radio-quiet quasar populations. Sources are compiled from archival data of both radio-loud and radio-quiet quasars over redshifts 0.1 < z < 5.0 to produce a sample of 353 sources with known X-ray, radio, and black hole mass measurements. A Fundamental Plane of accretion activity is fit to a sample of radio-loud and radio-quiet quasars, and we find a dichotomy between radio-loud and radio-quiet sources. The set of best-fitting equations that best describe the two samples are log LR = (1.12 ± 0.06)log LX − (0.20 ± 0.07)log M − (5.64 ± 2.99) for our radio-loud sample and log LR = (0.48 ± 0.06)log LX + (0.50 ± 0.08)log M + (15.26 ± 2.66) for our radio-quiet sample. Our results suggest that the average radio-quiet quasar emission is consistent with advection-dominated accretion, while a combination of jet and disc emission dominates in radio-loud quasars. We additionally examine redshift trends amongst the radio-loud and radio-quiet samples, and we observe a redshift dependence for the Fundamental Plane of radio-loud quasars. Lastly, we utilize the Fundamental Plane as a black hole mass estimation method and determine it useful in studying systems where standard spectral modelling techniques are not viable.

The X-Ray Coronae in NuSTAR Bright Active Galactic Nuclei

We present a systematic and uniform analysis of NuSTAR data of a sample of 60 SWIFT BAT-selected AGNs with 10–78 keV signal-to-noise ratio (S/N) > 50, 10 of which are radio loud. We measure their high-energy cutoff E cut or coronal temperature T e using three different spectral models to fit their NuSTAR spectra and show that a threshold in NuSTAR spectral S/N is essential for such measurements. High-energy spectral breaks are detected in the majority of the sample, and for the rest, strong constraints on E cut or T e are obtained. Strikingly, we find extraordinarily large E cut lower limits (>400 keV, up to >800 keV) in 10 radio-quiet sources, whereas we find none in the radio-loud sample. Consequently and surprisingly, we find a significantly larger mean E cut/T e of radio-quiet sources compared with radio-loud ones. The reliability of these measurements is carefully inspected and verified with simulations. We find a strong positive correlation between E cut and photon index Γ, which cannot be attributed to the parameter degeneracy. The strong dependence of E cut on Γ, which could fully account for the discrepancy of the E cut distribution between radio-loud and radio-quiet sources, indicates that the X-ray coronae in AGNs with steeper hard X-ray spectra have on average higher temperature and thus smaller opacity. However, no prominent correlation is found between E cut and λ edd. In the l–Θ diagram, we find a considerable fraction of sources lie beyond the boundaries of forbidden regions due to runaway pair production, posing (stronger) challenges to various (flat) coronal geometries.

Optical Singly-Ionized Iron Emission in Radio-Quiet and Relativistically Jetted Active Galactic Nuclei

The issue of the difference between optical and UV properties of radio-quiet and radio-loud (relativistically “jetted”) active galactic nuclei (AGN) is a long standing one, related to the fundamental question of why a minority of powerful AGN possess strong radio emission due to relativistic ejections. This paper examines a particular aspect: the singly-ionized iron emission in the spectral range 4400–5600 Å, where the prominent HI Hβ and [Oiii]λλ4959,5007 lines are also observed. We present a detailed comparison of the relative intensity of Feii multiplets in the spectral types of the quasar main sequence where most jetted sources are found, and afterwards discuss radio-loud narrow-line Seyfert 1 (NLSy1) nuclei with γ-ray detection and with prominent Feii emission. An Feii template based on I Zw 1 provides an accurate representation of the optical Feii emission for RQ and, with some caveats, also for RL sources. CLOUDY photoionization simulations indicate that the observed spectral energy distribution can account for the modest Feii emission observed in composite radio-loud spectra. However, spectral energy differences alone cannot account for the stronger Feii emission observed in radio-quiet sources, for similar physical parameters. As for RL NLSy1s, they do not seem to behave like other RL sources, likely because of their different physical properties, that could be ultimately associated with a higher Eddington ratio.

Radio loudness and classification for radio sources

Abstract The extragalactic radio sources are divided into two subclasses (radio-loud and radio-quiet sources) in the literature using radio loudness (R), which is defined as the ratio of radio emission to optical emission, but the boundary R-value separating the two classes is different in various sources. In this work, a sample of 2419 objects from the 13th catalog of quasars and active nuclei is used to build a boundary for the two subclasses. To do so, we compiled the radio and optical data, calculated their radio and optical indexes, made K-correction, obtained the radio loudness, and adopted a Bayesian analysis method to the logarithm of radio loudness for classification. We also investigated the correlations of radio loudness with radio/optical luminosities. Our main conclusions are summarized as follows: (1) The distribution of the logarithm of radio loudness (log R) is bimodal, the sources with log R < 1.26 are classified as radio-quiet sources, and those with log R > 1.26 are classified as radio-loud ones from the Bayesian analysis method. (2) The average radio-optical effective spectral index of radio-quiet sources is 〈αRO〉 = 0.05, while that of radio-loud sources is 〈αRO〉 = 0.55. (3) There are positive correlations between radio luminosity and radio loudness for both radio-loud sources and radio-quiet sources. (4) A dividing line of separating the distribution of the clusters on the diagram of radio loudness against radio luminosity was obtained statistically to set the boundary between radio-loud sources and radio-quiet sources, with an accuracy of $99.73\%$ based on the classification result from the Bayesian analysis method.

NuSTAR Hard X-Ray Spectra of Radio Galaxies

Abstract The Nuclear Spectroscopic Telescope Array (NuSTAR) observatory, with its 3–78 keV broadband spectral coverage, enables the detection of the high-energy cutoff in a number of active galaxies, including several individual radio-loud ones. In this work, we present systematic and uniform analyses of 55 Nuclear Spectroscopic Telescope Array (NuSTAR) spectra for a large sample of 28 radio galaxies, 20 of which are FR II galaxies. We perform spectral fitting to measure the high-energy cutoff E cut, photon index Γ, reflection factor R, and Fe Kα line equivalent width (EW). Measurements of E cut are given for 13 sources, and lower limits are given for the rest. We find that those E cut nondetections could primarily be attributed to the obviously smaller net photon counts in their spectra. This indicates that the NuSTAR spectra of the majority of our sample are dominated by the thermal coronal emission, and the E cut distribution of the sample is indistinguishable from that of a radio-quiet one in the literature. The flatter NuSTAR spectra we observed, comparing with radio-quiet sources, are thus unlikely due to jet contamination. The radio galaxies also show weaker X-ray reflection (both in R and Fe Kα line EW) compared with radio-quiet ones. Combining with the radio-quiet sample, we see a correlation between R and EW but with considerably large scatter. Notably, the radio-loud and -quiet sources appear to follow a common Γ–R correlation trend, supporting the outflowing corona model for both populations in which higher bulk outflowing velocity yields weaker reflection and flatter X-ray slope.

Probing the origin of low-frequency radio emission in PG quasars with the uGMRT – I

ABSTRACT We present the results from 685 MHz observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) of 22 quasars belonging to the Palomar-Green (PG) quasar sample. Only four sources reveal extended radio structures on ∼10–30 kpc scales, while the rest are largely a combination of a radio core unresolved at the uGMRT resolution of ∼3–5 arcsec, surrounded by diffuse emission on few kpc to ∼10 kpc scales. A few sources reveal signatures of barely resolved jets and lobes in their spectral index images that are created using the 685 MHz uGMRT data and similar resolution GHz-frequency data from the Very Large Array. On the basis of their position on the radio–IR correlation as well as the spectral index images, we find that the radio emission in the two radio-loud (RL) quasars and nearly one-third of the radio-quiet (RQ) quasars is active galactic nucleus (AGN) dominated whereas the remaining sources appear to have significant contributions from stellar-related processes along with the AGN. While the two RL sources exhibit inverted spectral index in their cores, the RQ sources exhibit a range of spectral indices varying from flat to steep (−0.1 $\gtrsim $ αR $\gtrsim $ −1.1) indicating the presence of unresolved jets/lobes or winds. Except for a significant correlation between the 685 MHz radio luminosity and the Eddington ratio, we do not find strong correlations between other 685 MHz radio properties and black hole (BH) properties in the RQ PG sources. This lack of correlations could be explained by the contribution of stellar-related emission, or radio emission from previous AGN activity episodes that may not be related to the current BH activity state.

AGN dichotomy beyond radio loudness: a Gaussian mixture model analysis

ABSTRACT Since the discovery of quasi-stellar objects (QSOs), also known as quasars, they have been traditionally subdivided as radio-loud and radio-quiet sources. Whether such division is a misleading effect from a highly heterogeneous single population of objects, or real has yet to be answered. Such dichotomy has been evidenced by observations of the flux ratio between the optical and radio emissions (usually B band and 5 GHz). Evidence of two populations in quasars and samples of a wide diversity of active galactic nuclei (AGNs) has been accumulated over the years. Other quantities beyond radio loudness also seem to show the signature of the existence of two different populations of AGNs. To verify the existence of a dichotomy through different parameters, we employed a soft clustering scheme, based on the Gaussian mixture model (GMM), to classify these objects simultaneously using the following parameters: black hole mass, colour, and R loudness index, as well as the usual radio and B-band luminosity. To investigate whether different kinds of AGNs manifest any population dichotomy, we applied GMM to four independent catalogues composed of both optical and radio information. Our results indicate the persistence of a dichotomy in all data sets, although the discriminating power differs for different choices of parameters. Although the radio loudness parameter alone does not seem to be enough to display the dichotomy, the evidence of two populations of AGNs could persist even if we consider other parameters. Our research suggests that the dichotomy is not a misleading effect but real.

Fundamental differences in the radio properties of red and blue quasars: enhanced compact AGN emission in red quasars

ABSTRACT We have recently used the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey to show that red quasars have fundamentally different radio properties to typical blue quasars: a significant (factor ≈3) enhancement in the radio-detection fraction, which arises from systems around the radio-quiet threshold with compact (<5 arcsec) radio morphologies. To gain greater insight into these physical differences, here we use the DR14 Sloan Digital Sky Survey (SDSS) and more sensitive, higher resolution radio data from the Very Large Array (VLA) Stripe 82 (S82) and VLA-COSMOS 3 GHz (C3GHz) surveys. With the S82 data, we perform morphological analyses at a resolution and depth three times that of the FIRST radio survey, and confirm an enhancement in radio-faint and compact red quasars over typical quasars; we now also find tentative evidence for an enhancement in red quasars with slightly extended radio structures (16–43 kpc at z = 1.5). These analyses are complemented by C3GHz, which is deep enough to detect radio emission from star-formation processes. From our data we find that the radio enhancement from red quasars is due to AGN activity on compact scales (≲43 kpc) for radio-intermediate–radio-quiet sources (−5 < $\mathcal {R}$ < −3.4, where $\mathcal {R}$ = $L_{\rm{1.4\,GHz}}/L_{6\mu\text{m}}$), which decreases at $\mathcal {R}$ < −5 as the radio emission from star-formation starts to dilute the AGN component. Overall our results argue against a simple orientation scenario and are consistent with red quasars representing a younger, earlier phase in the overall evolution of quasars.

Dichotomy of radio loud and radio quiet quasars in four dimensional eigenvector one (4DE1) parameter space

AbstractRecent work has shown that it is possible to systematize quasars (QSOs) spectral diversity in 4DE1 parameter space. The spectra contained in most of the surveys have low signal to noise ratio which fed the impression that all QSO’s are spectroscopically similar. Exploration of 4DE1 parameter space gave rise to the concept of two populations of QSOs that present important spectroscopic differences. We aim to quantify broad emission line differences between radio quiet and radio loud sources by exploiting more complete samples of QSO with spectral coverage in Hβ, MgII and CIV emission lines. We used a high redshift sample (0.35 < z < 1) of strong radio emitter QSOs observations from Calar Alto Observatory in Spain.

Probing narrow-line Seyfert 1 galaxies in the southern hemisphere

We present a new accurate catalog of narrow-line Seyfert 1 galaxies (NLS1s) in the southern hemisphere from the Six-degree Field Galaxy Survey (6dFGS) final data release, which is currently the most extensive spectroscopic survey available in the southern sky whose database has not yet been systematically explored. We classified 167 sources as NLS1s based on their optical spectral properties. We derived flux-calibrated spectra for the first time that the 6dFGS does not provide. By analyzing these spectra, we obtained strong correlations between the monochromatic luminosity at 5100 Å and the luminosities of Hβ and [O III]λ5007 lines. The central black hole mass and the Eddington ratio have average values of 8.6 × 106M⊙ and 0.96 LEdd respectively, which are typical values for NLS1s. In the sample, 23 (13.8%) NLS1s were detected at radio frequencies, and 12 (7.0%) of them are radio-loud. Our results confirmed that radio-loud sources tend to have higher redshift, a more massive black hole, and higher radio and optical luminosities than radio-quiet sources.

Large-scale environments of narrow-line Seyfert 1 galaxies

Studying large-scale environments of narrow-line Seyfert 1 (NLS1) galaxies gives a new perspective on their properties, particularly their radio loudness. The large-scale environment is believed to have an impact on the evolution and intrinsic properties of galaxies, however, NLS1 sources have not been studied in this context before. We have a large and diverse sample of 1341 NLS1 galaxies and three separate environment data sets constructed using Sloan Digital Sky Survey. We use various statistical methods to investigate how the properties of NLS1 galaxies are connected to the large-scale environment, and compare the large-scale environments of NLS1 galaxies with other active galacic nuclei (AGN) classes, for example, other jetted AGN and broad-line Seyfert 1 (BLS1) galaxies, to study how they are related. NLS1 galaxies reside in less dense environments than any of the comparison samples, thus confirming their young age. The average large-scale environment density and environmental distribution of NLS1 sources is clearly different compared to BLS1 galaxies, thus it is improbable that they could be the parent population of NLS1 galaxies and unified by orientation. Within the NLS1 class there is a trend of increasing radio loudness with increasing large-scale environment density, indicating that the large-scale environment affects their intrinsic properties. Our results suggest that the NLS1 class of sources is not homogeneous, and furthermore, a considerable fraction of them are misclassified. We further support a published proposal to replace the traditional classification to radio-loud, and radio-quiet or radio-silent sources with a division to jetted and non-jetted sources.

AGN spectral states from simultaneous UV and X-ray observations byXMM-Newton

The supermassive black holes in active galactic nuclei (AGN) and stellar-mass black holes in X-ray binaries (XRBs) are believed to work in a similar way. While XRBs evolve rapidly and several sources have undergone a few complete cycles from quiescence to an outburst and back, most AGN remain in the same state over periods of decades, due to their longer characteristic timescale proportional to their size. However, the study of the AGN spectral states is still possible with a large sample of sources. Multi-wavelength observations are needed for this purpose since the AGN thermal disc emission dominates in the ultraviolet energy range, while the up-scattered hot-corona emission is detected in X-rays. We compared simultaneous UV and X-ray measurements of AGN obtained by the XMM-Newton satellite. The non-thermal flux was constrained from the 2-12 keV X-ray luminosity, while the thermal disc component was estimated from the UV flux at 2900A. The hardness (ratio between the X-ray and UV plus X-ray luminosity) and the total luminosity were used to construct the AGN state diagrams. For sources with reliable mass measurements, the Eddington ratio was used instead of the total luminosity. The state diagrams show that the radio-loud sources have on average higher hardness, due to the lack of the thermal disc emission in the UV band, and have flatter intrinsic X-ray spectra. In contrast, the sources with high luminosity and low hardness are radio-quiet AGN. The hardness-Eddington ratio diagram reveals that the average radio-loudness is stronger for low-accreting sources, while it decreases when the accretion rate is close to the Eddington limit. Our results indicate that the general properties of AGN accretion states are similar to those of X-ray binaries. This suggests that the AGN radio dichotomy of radio-loud and radio-quiet sources can be explained by the evolution of the accretion states.

Optical Variability of Narrow-line and Broad-line Seyfert 1 Galaxies

Abstract We studied the optical variability (OV) of a large sample of narrow-line Seyfert 1 (NLSy1) and broad-line Seyfert 1 (BLSy1) galaxies with z < 0.8 to investigate any differences in their OV properties. Using archival optical V-band light curves from the Catalina Real Time Transient Survey that span 5–9 years and modeling them using damped random walk, we estimated the amplitude of variability. We found that NLSy1 galaxies as a class show lower amplitude of variability than their broad-line counterparts. In the sample of both NLSy1 and BLSy1 galaxies, radio-loud sources are found to have higher variability amplitude than radio-quiet sources. Considering only sources that are detected in the X-ray band, NLSy1 galaxies are less optically variable than BLSy1 galaxies. The amplitude of variability in the sample of both NLSy1 and BLSy1 galaxies is found to be anti-correlated with Fe ii strength but correlated with the width of the Hβ line. The well-known anti-correlation of variability–luminosity and the variability–Eddington ratio is present in our data. Among the radio-loud sample, variability amplitude is found to be correlated with radio-loudness and radio-power, suggesting that jets also play an important role in the OV in radio-loud objects, in addition to the Eddington ratio, which is the main driving factor of OV in radio-quiet sources.

Magneto‐rotational and thermal evolution of young neutron stars

AbstractAfter a brief review of population synthesis of close‐by cooling neutron stars, I focus on the interpretation of the dichotomy of spin periods of near‐by coolers. The existence of two well separated groups – short period (∼ 0.1–0.3 s) radio pulsars and long period (∼ 3–10 s) radio quiet sources, aka the Magnificent Seven, – can not easily be explained by unified models developed recently (Popov et al. 2010; Gullón et al. 2014). I speculate that the most natural solution of the problem can be in the bimodal initial magnetic‐field distribution related to the existence of an additional mechanism of field generation in magnetars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The faint radio source population at 15.7 GHz – II. Multi-wavelength properties

A complete, flux density limited sample of 96 faint ($> 0.5$ mJy) radio sources is selected from the 10C survey at 15.7 GHz in the Lockman Hole. We have matched this sample to a range of multi-wavelength catalogues, including SERVS, SWIRE, UKIDSS and optical data; multi-wavelength counterparts are found for 80 of the 96 sources and spectroscopic redshifts are available for 24 sources. Photometric reshifts are estimated for the sources with multi-wavelength data available; the median redshift of the sample is 0.91 with an interquartile range of 0.84. Radio-to-optical ratios show that at least 94 per cent of the sample are radio loud, indicating that the 10C sample is dominated by radio galaxies. This is in contrast to samples selected at lower frequencies, where radio-quiet AGN and starforming galaxies are present in significant numbers at these flux density levels. All six radio-quiet sources have rising radio spectra, suggesting that they are dominated by AGN emission. These results confirm the conclusions of Paper I that the faint, flat-spectrum sources which are found to dominate the 10C sample below $\sim 1$ mJy are the cores of radio galaxies. The properties of the 10C sample are compared to the SKADS Simulated Skies; a population of low-redshift starforming galaxies predicted by the simulation is not found in the observed sample.

The naked nuclei of low ionization nuclear emission line regions

We analyze HST spectra and Chandra observations of a sample of 21 low ionization nuclear emission line regions (LINERs), at least 18 of which genuine active galactic nuclei. We find a correlation between the X-rays and emission lines luminosities, extending over three orders of magnitude and with a dispersion of 0.36 dex; no differences emerge between LINERs with and without broad lines, or between radio-loud and radio-quiet sources. The presence of such a strong correlation is remarkable considering that for half of the sample the X-ray luminosity can not be corrected for local absorption. This connection is readily understood since the X-ray light is associated with the same source producing the ionizing photons at the origin of the line emission. This implies that we have a direct view of the LINERs nuclei in the X-rays: the circumnuclear, high column density structure (the torus) is absent in these sources. Such a conclusion is also supported by mid-infrared data. We suggest that this is due to the general paucity of gas and dust in their nuclear regions that causes also their low rate of accretion and low bolometric luminosity.

A low-luminosity type-1 QSO sample

We report on the optical spectroscopic analysis of a sample of 99 low-luminosity quasi-stellar objects (LLQSOs) at $z\leq 0.06$ base the Hamburg/ESO QSO survey (HES). The LLQSOs presented here offer the possibility of studying the faint end of the QSO population at smaller cosmological distances and, therefore, in greater detail. A small number of our LLQSO present no broad component. Two sources show double broad components, whereas six comply with the classic NLS1 requirements. As expected in NLR of broad line AGNs, the [S{\sc{ii}}]$-$based electron density values range between 100 and 1000 N$_{e}$/cm$^{3}$. Using the optical characteristics of Populations A and B, we find that 50\% of our sources with H$\beta$ broad emission are consistent with the radio-quiet sources definition. The remaining sources could be interpreted as low-luminosity radio-loud quasar. The BPT-based classification renders an AGN/Seyfert activity between 50 to 60\%. For the remaining sources, the possible star burst contribution might control the LINER and HII classification. Finally, we discuss the aperture effect as responsible for the differences found between data sets, although variability in the BLR could play a significant role as well.

A survey of extended H2 emission from massive YSOs

We present the results from a survey, designed to investigate the accretion process of massive young stellar objects (MYSOs) through near infrared narrow band imaging using the H$_2$ $\nu$=1-0 S(1) transition filter. A sample of 353 Massive Young Stellar Object (MYSO) candidates was selected from the Red MSX Source survey using photometric criteria at longer wavelengths (infrared and submillimeter) and chosen with positions throughout the Galactic Plane. Our survey was carried out at the SOAR Telescope in Chile and CFHT in Hawaii covering both hemispheres. The data reveal that extended H$_2$ emission is a good tracer of outflow activity, which is a signpost of accretion process on young massive stars. Almost half of the sample exhibit extended H$_2$ emission and 74 sources (21\%) have polar morphology, suggesting collimated outflows. The polar-like structures are more likely to appear on radio-quiet sources, indicating these structures occur during the pre-UCHII phase. We also found an important fraction of sources associated with fluorescent H$_2$ diffuse emission that could be due to a more evolved phase. The images also indicate only $\sim$23\% (80) of the sample is associated with extant (young) stellar clusters. These results support the scenario in which massive stars are formed by accretion disks, since the merging of low mass stars would not produce outflow structures.