Showing papers in "The Astrophysical Journal in 1983"
TL;DR: In this paper, the authors considered the possibility that there is not much hidden mass in galaxies and galaxy systems and proposed a modified version of the Newtonian dynamics to describe the motion of bodies in a gravitational field (of a galaxy, say).
Abstract: The author considers the possibility that there is not, in fact, much hidden mass in galaxies and galaxy systems. If a certain modified version of the Newtonian dynamics is used to describe the motion of bodies in a gravitational field (of a galaxy, say), the observational results are reproduced with no need to assume hidden mass in appreciable quantities. Various characteristics of galaxies result with no further assumptions. The basis of the modification is the assumption that in the limit of small acceleration a very low a0, the acceleration of a particle at distance r from a mass M satisfies approximately a2/a0 a MGr-2, where a0 is a constant of the dimensions of an acceleration.
3,094 citations
TL;DR: In this article, the absolute spectral energy distribution for the primary standard star Alpha Lyrae was adopted and absolute fluxes for the four very metal-deficient F type subdwarfs HD 19445, HD 84937, BD + 26.2606 deg, and BD + 17.4703 deg.
Abstract: Based on an adopted absolute spectral energy distribution for the primary standard star Alpha Lyrae, absolute fluxes are given for the four very metal-deficient F type subdwarfs HD 19445, HD 84937, BD + 26.2606 deg, and BD + 17.4703 deg. Somewhat inferior data are also given for HD 140283. The data are given for 40-A bands and cover the wavelength range from 3080 A to 12,000 A. The four stars, all near magnitude 9 and distributed around the sky, are intended as secondary standards for absolute spectrophotometry.
2,464 citations
TL;DR: In this article, the effective radii of the Roche lobes were computed and compared with the results of Kopal and Pacynski, and a convenient approximation was given, whose derivative was smooth and fairly accurate.
Abstract: Effective radii of Roche lobes were computed and are compared with the results of Kopal and of Pacynski. A convenient approximation is given, whose derivative is smooth and fairly accurate.
2,141 citations
TL;DR: An effective absorption cross section per hydrogen atom has been calculated as a function of energy in the 0.03-10 keV range using the most recent atomic cross section and cosmic abundance data.
Abstract: An effective absorption cross section per hydrogen atom has been calculated as a function of energy in the 0.03-10 keV range using the most recent atomic cross section and cosmic abundance data. Coefficients of a piecewise polynomial fit to the numerical results are given to allow convenient application in automated calculations.
1,967 citations
TL;DR: In this paper, two formulas for estimating the significance of positive observations in searching gamma-ray sources or lines were proposed for analyzing the results of Gamma-ray astronomy experiments, which were tested with the aid of Monte Carlo simulations.
Abstract: The procedures in current use for analyzing the results of gamma-ray astronomy experiments are assessed. Two formulas are proposed for estimating the significance of positive observations in searching gamma-ray sources or lines. The correctness of the formulas is tested with the aid of Monte Carlo simulations. One formula is derived by immediately estimating the standard deviation of the observed signal NS; the other is derived by applying the method of the statistical hypotheses test.
1,211 citations
TL;DR: In this paper, the authors used the distance information provided by the redshifts to estimate the two-point spatial correlation function zeta(r) in a way that is designed to be unbiased by peculiar velocities.
Abstract: We describe the results of a study of the two-point correlations in the 14.5 m/sub B/ CfA redshift survey. We use the distance information provided by the redshifts to estimate the two-point spatial correlation function zeta(r) in a way that is designed to be unbiased by peculiar velocities. The results agree well with what has been found from the deeper angular distributions. In the fiducial model zeta(r) = (r/sub 0//r)/sup ..gamma../ with ..gamma.. = 1.77 we find r/sub 0/ = 5.4 +- 0.3 h/sup -1/ Mpc (H/sub 0/ = 100 h km s/sup -1/ Mpc/sup -1/). At r> or approx. =10 h/sup -1/ Mpc, zeta(r) seems to steepen and may in fact be negative at 20< or approx. =hr< or approx. =40 Mpc. In existing n-body simulations zeta(r) is poorly modeled by a power law, with more power on small scales and less power on large scales than the data. This confirms the visual impressions that the n-body clusters are too compact and the clusters too homogeneously distributed relative to the data.
1,202 citations
TL;DR: In this article, the photoionization properties of a stellar population have been modeled for a variety of choices for the initial mass function (IMF) and the observed UBV colors and H..cap alpha.. emission equivalent widths place tight constraints on the slope of the IMF between 1 M /sub sun/ and 50 M / sub sun/ in the galaxies.
Abstract: Photometry of the integrated H..cap alpha.. emission in a large sample of field spiral and irregular galaxies has been used to obtain quantitative estimates of the total star formation rate (SFR) in the galaxies. The photoionization properties of a stellar population have been modeled for a variety of choices for the initial mass function (IMF). The observed UBV colors and H..cap alpha.. emission equivalent widths place tight constraints on the slope of the IMF between 1 M /sub sun/ and 50 M /sub sun/ in the galaxies; excellent agreement with the observed galaxy colors and H..cap alpha.. emission is obtained with models using an IMF slope close to Salpeter's original value. The properties of late-type galaxies are not well reproduced by the Miller-Scalo solar neighborhood IMF. The extinction-corrected star formation rates are large, as high as 20 M /sub sun/ yr/sup -1/ in giant Sc galaxies (H/sub 0/ = 50 km s/sup -1/ Mpc/sup -1/). The current rates in late-type galaxies are comparable to the past rates averaged over the age of the disk; late-type disk galaxies have evolved at a nearly constant rate, confirming earlier models by Searle, Sargent, and Bagnuolo. Little evidence is found for a strong correlationmore » between the SFR and average gas density; if the SFR proportional rho/sup n/, then the exponent n must be much less than 1, corroborating earlier studies of star formation in the solar neighborhood by Miller, Scalo, and Twarog. Comparison of the present SFRs with the remaining supply of interstellar gas yields consumption time scales of only a few times 10/sup 9/ years in most cases, in agreement with the model estimates of Larson, Tinsley, and Caldwell.« less
1,031 citations
1,028 citations
TL;DR: The Bright Quasar Survey (BQS) as mentioned in this paper consists of 114 objects to an average limiting magnitude B = 16.16 over an area of 10,714 deg/sup 2.
Abstract: We present the Bright Quasar Survey (BQS) consisting of 114 objects to an average limiting magnitude B = 16.16 over an area of 10,714 deg/sup 2/. There are 92 quasars with M/sub B/<-23 in the sample. We use the BQS and complete samples from published surveys to derive models of the statistical evolution of quasars. The increase of space density with redshift depends strongly on absolute luminosity, being close to zero for low-luminosity quasars. Detailed predictions are given for the distribution of redshifts and magnitudes and for the total counts based on the evolution models.
1,000 citations
716 citations
TL;DR: In this paper, an upper limit to the spectrum of the isotropic gravitational radiation background has been derived in the frequency band 4 x 10 to the -9th to 10 to -7th Hz.
Abstract: A pulsar and the earth may be thought of as end masses of a free-mass gravitational wave antenna in which the relative motion of the masses is monitored by observing the Doppler shift of the pulse arrival times. Using timing residuals from PSR 1133 + 16, 1237 + 25, 1604-00, and 2045-16, an upper limit to the spectrum of the isotropic gravitational radiation background has been derived in the frequency band 4 x 10 to the -9th to 10 to the -7th Hz. This limit is found to be S(E) = 10 to the 21st f-cubed ergs/cu cm Hz, where S(E) is the energy density spectrum and f is the frequency in Hz. This would limit the energy density at frequencies below 10 to the -8th Hz to be 0.00014 times the critical density.
TL;DR: The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed in this article, and the different types of shock solutions, and the circumstances under which the different kinds occur, are delineated.
Abstract: The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C*- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics
TL;DR: In this paper, periodic pulsations have been sought in the NGC 1851, 6441, 6624, and 6712 globular cluster sources' X-ray emission, using the Einstein Observatory's Monitor Proportional Counter.
Abstract: Periodic pulsations have been sought in the NGC 1851, 6441, 6624, and 6712 globular cluster sources' X-ray emission, using the Einstein Observatory's Monitor Proportional Counter. Special attention is given to features of the present methods of analysis, which correctly account for several effects that have been routinely overlooked by other researchers. No pulsed emission was detected in the periods searched, which covered from 1 msec to about 500 sec. The 90% confidence upper limits for the pulsed fraction are given.
TL;DR: In this paper, a simplified stellar evolution code is proposed to calculate the evolution of close binary systems with collapsed binaries and mass-losing secondaries, which is a more general, but still simplified, technique for calculating the evolution.
Abstract: The development of appropriate computer programs has made it possible to conduct studies of stellar evolution which are more detailed and accurate than the investigations previously feasible. However, the use of such programs can also entail some serious drawbacks which are related to the time and expense required for the work. One approach for overcoming these drawbacks involves the employment of simplified stellar evolution codes which incorporate the essential physics of the problem of interest without attempting either great generality or maximal accuracy. Rappaport et al. (1982) have developed a simplified code to study the evolution of close binary stellar systems composed of a collapsed object and a low-mass secondary. The present investigation is concerned with a more general, but still simplified, technique for calculating the evolution of close binary systems with collapsed binaries and mass-losing secondaries.
TL;DR: In this article, the authors explored the consequences of this general dynamical dissipation in the magnetic fields that produce the active corona of the Sun and showed that the footpoints of the field are continually manipulated by the subphotospheric convection, so that the lines of force are continually wrapped and rotated about each other.
Abstract: It is shown in the previous paper that whenever twisted flux tubes are bundled together, they are subject to dynamical nonequilibrium and internal neutral point reconnection, causing rapid dissipation of their torsion. In the present paper we explore the consequences of this general dynamical dissipation in the magnetic fields that produce the active corona of the Sun. The footpoints of the field are continually manipulated by the subphotospheric convection, so that the lines of force are continually wrapped and rotated about each other. The dynamical dissipation of the wrapping and rotation transfers the work done on the footpoints directly into heat in the corona, at a rate estimated to be of the order of 10/sup 7/ ergs cm/sup -2/ s/sup -1/. The effect appears to be the principal source of heat to the visible corona. This general picture implies that all magnetic fields extending outward from convecting astronomical bodies produce intense heating of the tenuous outer atmospheres of these bodies, in general agreement with the observed fact of the universal activity of stars and galaxies.
TL;DR: In this paper, the spectral evolution of early-type galaxies using population synthesis techniques is studied, where different choices of the initial mass function and the star formation rare allow the construction of models which at the present epoch cover the range of observed galaxy spectra.
Abstract: This work develops models for the spectral evolution of early-type galaxies using population synthesis techniques. Different choices of the initial mass function and the star formation rare allow the construction of models which at the present epoch cover the range of observed galaxy spectra. Special emphasis is given to the ultraviolet region of the spectrum. Ultraviolet spectra of late-type stars and early-type galaxies obtained with the International Ultraviolet Explorer satellite are used in this work. The evolving spectral energy distributions are used to compute galaxy colors, magnitudes, K-corrections, and evolutionary corrections as functions of redshift for different photometric systems.
TL;DR: In this article, a unified description of the properties of 14 X-ray pulsars is presented and compared with the current theoretical understanding of these systems, with the only trend in the phase averaged spectra being that the lower luminosity systems appear to have less abrupt high energy cutoffs.
Abstract: A unified description of the properties of 14 X-ray pulsars is presented and compared with the current theoretical understanding of these systems. The sample extends over six orders of magnitude in luminosity, with the only trend in the phase averaged spectra being that the lower luminosity systems appear to have less abrupt high energy cutoffs. There is no correlation of luminosity with power law index, high energy cutoff energy or iron line EW. Detailed pulse phase spectroscopy is given for five systems.
TL;DR: In this article, it was shown that a chemically significant flux of UV photons may exist inside dense clouds due to cosmic ray excitation of the various band systems of hydrogen, and that these photons would recover C I from the CO reservoir in dense clouds via photodissociation at an order of magnitude faster rate than that possible in connection with the reaction of CO with He(+).
Abstract: Interstellar UV radiation cannot penetrate into the interior of dense clouds, and cosmic ray ionization is thought to be the sole driver of the gas phase chemistry. However, cosmic ray energy deposition also involves electronic excitation of the absorbing gas. It appears, therefore, possible that emissions resulting from these excitations might maintain a significant flux of chemically effective UV photons in the interior of dense clouds. The present investigation is concerned with this possibility, taking into account a simplified approach. This approach involves a consideration of Lyman and Werner band photons of molecular hydrogen and their relevance to C and CO chemistry. The results of this investigation suggest that a chemically significant flux of UV photons may exist inside dense clouds due to cosmic ray excitation of the various band systems of hydrogen. These photons would recover C I from the CO reservoir in dense clouds via photodissociation at an order of magnitude faster rate than that possible in connection with the reaction of CO with He(+).
TL;DR: In this paper, the transition of branch giants of 0.8 and 1.0 M/sub sun/ into the white dwarf configuration was studied and lower limits for mass and luminosity of central stars of planetary nebulae were derived.
Abstract: We have computed the transition of asymptotic branch giants of 0.8 and 1.0 M/sub sun/ into remnants of 0.565, 0.553, and 0.546 M/sub sun/ by means of mass loss which removed the stellar envelopes in about 10/sup 3/ yr. The remnants were then evolved across the H-R diagram to the white dwarf configuration and their relevance for central stars of planetary nebulae studied. From the temporal evolution of these models we determined lower limits for mass and luminosity of central stars of approx.0.55 M/sub sun/ and approx.2500 L/sub sun/.
TL;DR: In this article, the mass loss rate as a function of distance from the source of X rays is estimated for Compton-heated coronae and winds, and the results have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables.
Abstract: X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.
TL;DR: In this article, the authors examined the stability of a thin spherical shell against dynamic and gravitational perturbations when the wavelength of the perturbation is large compared to the thickness of the dense shell.
Abstract: In this paper we examine the stability of a thin spherical shock against dynamic and gravitational perturbations when the wavelength of the perturbation is large compared to the thickness of the dense shell. For dynamic perturbations, we find that a spherical isothermal shock, driven by a hot gas, is overstable against small perturbations. As a result, the fragmentation of the shell proceeds in an oscillatory manner. The smaller wavelength perturbations are most unstable, with the maximum growth rate being comparable to the inverse of the time it takes sound waves to propagate through the shell thickness. The nonradiating case is not subject to this overstability unless d ln P/d in rho is close to 1. The overstability can appear in cases where there is an ambient magnetic field. We also consider the growth of gravitationally driven perturbations. On cosmological time scales all such shocks are unstable, with the density perturbations growing as power laws in time. In the nonradiating case the maximum exponent is close to 1. On shorter time scales, only the isothermal shocks are unstable. Whenever the overstability mechanism is at work, gravitational instabilities can be neglected. We discuss the implications of these results for star formation, andmore » for hydrodynamic galaxy formation.« less
TL;DR: In this article, a series of computer simulations of the solar modulation of galactic cosmic rays using a full three-dimensional model which incorporates all known important effects on particle transport, particle drifts, convection with the solar wind, energy loss, and anisotropic diffusion is presented.
Abstract: The first results are presented from a series of computer simulations of the solar modulation of galactic cosmic rays using a full three-dimensional model which incorporates all known important effects on particle transport, particle drifts, convection with the solar wind, energy loss, and anisotropic diffusion. The model is time-independent in the coordinate frame rotating with the sun, so corotating effects can be studied. Modulation in an interplanetary magnetic field model is considered in which the current sheet separating the northern and southern solar hemispheres is warped and corotating with the sun. The amplitude of the warp is varied to simulate possible solar cycle variation of the magnetic field. Substantial effects due to the warp of the current sheet are found. Comparison of the model results with various data is presented. Among other things, it is shown that the intensity may decrease away from the current sheet for both signs of the magnetic field, as suggested by recent observations, and in contrast with inferences from earlier, more approximate calculations.
TL;DR: In this article, a series of objective statistical estimators are applied to directly study the three-dimensional distribution of rich clusters, using a recently completed redshift sample of 104 Abell clusters out to distance class four and galactic latitude of 30 deg or more.
Abstract: A series of objective statistical estimators is applied to directly study the three-dimensional distribution of rich clusters, using a recently completed redshift sample of 104 Abell clusters out to distance class four and galactic latitude of 30 deg or more. The sample and the relevant positional and redshift selection effects are discussed, and the two-point angular correlation function and nearest-neighbor distribution of clusters in the sample are determined, as well as in the larger D = 5+6 sample and in several subsamples of different richness classes and latitude zones. The redshift correlation function of clusters for various angular separations on the sky is examined as a test for the reality of the observed angular correlations. The two-point spatial correlation function of clusters in the sample is determined, and the consistency among the three observed correlation functions is tested. A model that describes all the observed correlations is presented.
TL;DR: In this paper, the authors present partial results of a survey of approx.100 visually opaque regions in nearby dark clouds in the 1.3 cm (J,K) = (1,1) line of NH/sub 3/, with mapping of all strong sources (''dense cores'') and (2,2) line observations in selected positions.
Abstract: We present partial results of a survey of approx.100 visually opaque regions in nearby dark clouds in the 1.3 cm (J,K) = (1,1) line of NH/sub 3/, with mapping of all strong sources (''dense cores'') and (2,2) line observations in selected positions. For 27 dense cores with distance estimates, mean properties are: FWHM diameter, 0.1 pc; density, 3 x 10/sup 4/ cm/sup -3/; mass, 4 M/sub sun/; kinetic temperature, 11 K; and FWHM velocity width, 0.3 km s/sup -1/. We compare line shapes with cloud motion models, and source density, size, and temperature with equilibrium and stability requirements. These indicate that most dense cores are in the early stages of collapse or in near-critical equilibrium; if in equilibrium, they are probably supported by a combination of thermal and subsonic turbulent motions. In Taurus-Auriga, positions of the 10 known dense cores are well correlated with positions of emission-line star groups. In the next dense core free-fall time, 2 x 10/sup 5/ yr, the Taurus-Auriga complex is expected to form 25--50 emission-line stars. This is consistent with the estimated number of dense cores, 25. Taken together, these results suggest that most of the dense cores described here will form low-mas stars inmore » the next approx.10/sup 6/ yr.« less
TL;DR: In this article, magnetograms made at Mt. Wilson Observatory from January 1967 to May 1982 are crosscorrelated in 34 latitude strips at 1-4-day increments to determine the rotation of magnetic features in the solar photosphere.
Abstract: Magnetograms made at Mt. Wilson Observatory from January 1967 to May 1982 are crosscorrelated in 34 latitude strips at 1-4-day increments to determine the rotation of magnetic features in the solar photosphere. The data are smoothed by averaging corresponding correlations and calculating rotation from the displacement of the averaged-correlation maximum; the usefulness and validity of this procedure are discussed. No significant time variation or field dependence is found for the period of the observations, at least to the accuracy of the calculated means (variance of from about 2 m/sec at low latitudes to about 10 m/sec near the poles). The rotation function omega at solar latitude phi is shown to be 2.902 0.464 sin sq phi - 0.328 sin to the 4th phi microrad/sec, in agreement with the Mt. Wilson Doppler profile near the poles and with the sunspot determination of Newton and Nunn (1951) at sunspot latitudes, where the Doppler estimate is about 30 m/sec slower.
TL;DR: In this paper, the authors investigated the thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas, where the cloud is assumed to be in pressure-free collapse.
Abstract: We investigate the thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas. The cloud is assumed to be in pressure-free collapse. Over a wide range of initial conditions, virtually all the gas is converted to molecular form by a density n = 10/sup 12/ cm/sup -3/. The most effective reactions are the three-body ones: H+H+H..-->..H/sub 2/+H and H+H+H/sub 2/..-->..2H/sub 2/. As a result of significant cooling from the molecules, the temperature rise is slowed, and the Jeans mass eventually falls below 0.1 M/sub sun/ for clouds less massive than 100 M/sub sun/. Such clouds should therefore be capable of fragmenting into low-mass stars. This conclusion is strengthened if angular momentum slows the collapse. We also include in a heuristic manner the effect of shock heating from colliding fragments in a turbulent collapsing cloud. With substantial heating, the Jeans mass cannot drop as far, owing to the early destruction of hydrogen molecules. The primordial stellar mass spectrum may therefore be a sensitive function of the degree and effectiveness of intercloud collisions.
TL;DR: In this article, a theory of pair creation over the polar caps of isolated, magnetized, rotating neutron stars with magnetic axis oblique to the rotation is developed, when steady, space charge limited flow occurs within a narrow polar flux tube bounded by regions of high conductivity and negligible flow.
Abstract: A theory of pair creation over the polar caps of isolated, magnetized, rotating neutron stars with magnetic axis oblique to the rotation is developed, when steady, space charge limited flow occurs within a narrow polar flux tube bounded by regions of high conductivity and negligible flow. A quantitative calculation is given of the structure of the pair formation front which divides the low altitude, charge separated region of strong acceleration from the dense pair plasma flowing outward along the polar flux tube, including the structure and energetics of the slot gap formed between the pair plasma and the closed regions of the magnetosphere. Both the electron emission and the ion emission cases are incorporated in the electrodynamical theory. In short period objects with simple magnetic structure near the star, the particle acceleration power generated is found to be on the order of a few tenths of a percent of the total spin-down energy loss, more than enough to explain all the photon emission from the Crab and Vela pulsars. Most of this energy is generated in the slot gap. In dipole geometry, the energy is concentrated into two or three ''beams'' of accelerated particles with the regions of highest energymore » density filling only a small fraction of the polar flux tube. In long period objects with complex surface fields, the power generated in particle acceleration is comparable to the total spin-down energy loss rate, and is mostly created in the low altitude gap region between the star and the pair plasma. Some implications of this local particle acceleration theory for the beaming morphology of pulsars' photon emission are discussed, especially the possibility of observationally distinguishing the electron emission and the ion emission cases.« less
TL;DR: In this article, it was shown that dense cores can form low-mass stars on free-fall time scales if their input of fresh turbulence is sufficiently reduced, which may occur as a result of their small size, or their high density more contrast with surrounding gas.
Abstract: In 43 dark clouds with narrow molecular lines, significant correlations exist between line width and map size of the form ..delta..vproportionalR/sup 0.5/, and between mean density and map size of the form nproportionalR/sup -1/; and the law of virial equilibrium is closely satisfied. These relations tend to confirm those found earlier for larger clouds, and extend them into the regime of subsonic turbulence. If these relations for large and small clouds reflect the same process, it is still unclear whether this process is a turbulent energy cascade, as in Kolmogorov turbulence, or simply the tendency of clouds with nproportionalR/sup -1/ to be in virial equilibrium. For the smallest clouds (''dense cores'') there is weak support for the turbulent cascade picture. In this picture viscous dissipation of turbulence is likely to play an important role in heating and star formation. The time scale for the free decay of turbulence in dense cores is < or approx. =5 x 10/sup 5/ yr, comparable to the free-fall time. Dense cores can thus form low-mass stars on free-fall time scales if their input of fresh turbulence is sufficiently reduced. This reduction may occur as a result of their small size, or their high densitymore » contrast with surrounding gas. Observed distributions of dense core properties in Taurus, Ophiuchus, and other dark cloud regions appear consistent with dense core evolution toward star formation via dissipation of turbulence. In complexes which are vigorously forming low-mass stars, dense cores are more prevalent, smaller, denser, and have narrower lines than in regions with less star formation.« less