! Hubble Space Telescope Cycle 6 (1996) Phase II Proposal Template
! $Id: 6672,v 7.1 1996/08/02 17:20:50 pepsa Exp $
!
!   Refer to the HST Phase II Proposal Instructions to fill this out
!
!   Anything after a "!" is ignored, and may be deleted
!
!   All keywords with multiple entries are comma delimited except the
!   Visit_Requirements and Special_Requirements keywords which can be
!   delimited with carriage returns or semi-colons, but not commas
!
! For help call your Program Coordinator: Karla Peterson
!             Phone: 410-338-4774    , E-mail: peterson@stsci.edu
!
! This partially completed template was generated from a Phase I proposal.


Proposal_Information                           
              Title:  Unveiling the massive star content in the
                      prototypical nuclear starburst NGC7714
  Proposal_Category:  GO
Scientific_Category:  GALAXIES & CLUSTERS
             Cycle:    6

Investigators
     PI_name:         Maria Luisa Garcia-Vargas
     PI_Institution:  VILSPA, IUE Observatory. ESA

          CoI_Name:   Jeff Goldader
   CoI_Institution:   Space Telescope Science Institute
           Contact:   N         

          CoI_Name:   Rosa Gonzalez-Delgado
   CoI_Institution:   Space Telescope Science Institute
           Contact:   N         

          CoI_Name:   Ariane Lanccon
   CoI_Institution:   Observatoire de Strasbourg
           Contact:   N         

          CoI_Name:   Claus Leitherer
   CoI_Institution:   Space Telescope Science Institute
           Contact:   N        

          CoI_Name:   Antonella Nota
   CoI_Institution:   Space Telescope Science Institute
           Contact:   N        

          CoI_Name:   Anna Pasquali
   CoI_Institution:   Space Telescope Science Institute
           Contact:   N          


Abstract:                                
    We propose ultraviolet spectroscopy with GHRS+G140L and UV
    WFPC2(F380W) imaging of the prototypical starburst galaxy NGC
    7714. The compact appearance and the brightness of the nuclear
    starburst, as well as the amount of multiwavelength
    observational data, lead us to propose this object as a
    laboratory to study the star formation processes in the
    violent nuclei of starburst galaxies. The Cycle 6 spectroscopy
    will allow us to derive the massive star content in the
    nuclear starburst of NGC 7714 directly from the signatures of
    the massive stars in the ultraviolet spectrum. These spectra
    will complete the multiwavelength body of data for this
    nuclear starburst. A complete evolutionary synthesis code will
    be used to interpret the data. These two complementary aspects
    (observational and theoretical) are the master keys to
    unveil the stellar content of star-forming regions and, in
    particular, to derive the age of the stellar population
    present in the nuclear starburst, and to constrain the slope
    and upper limit for the Initial Mass Function. Finally, the UV
    WFPC2 imaging will complete the study by permitting us to
    compare the locations of the UV and H-alpha (HST archive
    image) sources with the spatial distribution of massive stars
    in the nucleus and circumnuclear regions, constituting an
    additional observational constraint for the theoretical
    models.

Questions                       

     Observing_Description:
          GHRS spectroscopy: our goal is to obtain GHRS UV spectra of
         the starburst nucleus of NGC7714. These will be compared to
         our spectral synthesis models and used to determine the
         parameters of the massive star population (e.g., burst age,
         slope of the upper end of the IMF, upper cut-off mass). We
         need S/N = 30 per diode in the continuum to detect and model
         age- and IMF-sensitive stellar absorption lines. This value
         is based on our experience with previous modeling of  IUE and
         HST data (Robert et al. 1993; Vacca et al. 1995; Leitherer et
         al. 1995). Lower S/N makes it difficult to resolve spectral
         lines which are blends of stellar and interstellar
         contributions. We compared the capabilities of the FOS and
         G130H grating and of the GHRS and G140L grating. After taking
         into account instrumental overheads, sensitivity, scattered
         light in the far-UV, wavelength coverage, aperture sizes, and
         spectral resolution, we find that the GHRS+G140L is the
         instrument of choice. Above all, the most important strategic
         lines used to characterize the burst (C IV 1550, Si IV 1400,
         N V 1240) have wavelengths below 1600 Angstrom, where the
         GHRS sensitivity is clearly superior. The flux of the nucleus
         of NGC7714 is ~ 4* 10**-14 erg s**-1 cm**-2 AA**-1 at 1400 AA
         from the  IUE spectrum of Kinney et al. (1993).
         Note that the  IUE flux is entirely dominated by the nucleus
         despite the much larger aperture size of the  IUE. We
         determined the flux encompassed by the GHRS LSA aperture from
         pre-Costar  HST images in the Archive. About 50 % of the  IUE
         flux falls within the 1.7'' aperture of the GHRS. Moreover,
         the use of the 1''.7 arc sec aperture for spectroscopy,
         similar to ground observations will be useful to compare the
         WR content from ultraviolet fitting and from optical WR
         bumps). The chosen aperture will encompass the total UV light
         of the nuclear starburst in NGC 7714. In fact, from the
         analysis of the circumnuclear regions (Garcia-Vargas et al.
         1995c), and on the basis of evolutionary synthesis models
         (Garcia-Vargas, Bressan & Diaz 1995a,b) we derive that
         the contribution of the circumnuclear emission to the UV flux
         is 2 order or magnitudes lower than the nuclear one. This
         implies that the UV spectrum is dominated by the nuclear
         starburst. Therefore, the long-slit capabilities of STIS
         would bring no real advantage. We need two grating positions
         to cover the essential wavelength region from 1200 AA 
         to 1700 AA. At the shorter wavelengths, S/N ~ 30 is
         reached in 1 orbit for each grating setting. The target
         acquisition can be done directly with the GHRS, i.e. the FOS
         is not needed. However, the acquisition has to be done with
         Side-2, as NGC7714 is too faint for a direct Side-1
         acquisition. This will be done in the first orbit, followed
         by a side switch partially buried in the occultation.  The
         grand total for the spectroscopy is therefore 3 orbits.
         
     WFPC2 imaging: 
          We selected the  WFPC2 passband
         F380W which is optimized for measurements of hot stars and
         has the advantage with respect to the standard F336W of
         minimizing the red leak contribution from the redder
         population which we know to be present in NGC7714.  A
         limiting V magnitude of 26 is sufficient at this wavelength
         to observe the hottest (brighter) stars. We will use the PC,
         to exploit the highest spatial resolution offered by WFPC2.
         In order to demonstrate the need for the requested exposure
         time we will consider the case of a B0 star with a V
         magnitude of 26. The predicted count rate discussed here has
         been calculated using Synphot, and the WWW WPC2 exposure time
         estimator, which are assumed to have the most up to date
         information on WFPC2's throughput. A B0 star with a V
         magnitude of 26 yields a source count rate of 0.11
         electrons/sec through the F380W filter. A single 800 second
         observation will produce an integrated flux of 85 counts.
         WFPC2 has a relatively low pixel modulation transfer function
         (MTF) which effectively results in significant smoothing of
         the stellar point spread function. Consequently,  the
         fraction of counts in the central PC pixel is 0.26 of the
         total count at a wavelength of 4000 Angstrom. The central
         pixel therefore contains  22 counts (WFPC2 Handbook). In
         addition to photon counting statistics there are additional
         noise contributions due to: 1) CCD read noise ~eq 7e-, 2) CCD
         dark current ~eq  0.005 e-/sec, 3) Sky background ~eq  0.005
         e/sec at 4000 Angstrom. For a single exposure, combining all
         noise contributions, we derive a value for the S/N in the
         brightest pixel of 2. We require 4 exposures to achieve a S/N
         of 4 in the central pixel. In addition to the signal to noise
         considerations, we require a minimum of 4 exposures to obtain
         4 CR-SPLIT images at two dither points. Dithering is a
         strategy  essential to remove systematics during analysis of
         the data. Therefore, we request a 4*800 seconds in order to
         meet our goal of  a limiting U magnitude of 26 in the F380W
         filter. With the assumption of an orbit lasting 55 minutes,
         our total request of WFPC2 imaging time is 1 orbit.

     Relation to theoretical models: 
          The fitting to the HST ultraviolet observations will be made
         with the use of the models by Leitherer, Robert & Heckman
         (1995), after a careful deblending to avoid interstellar
         contamination. The multiwavelengh match will be make with the
         use of three different synthesis codes which have been
         developed by members of our team (Garcia Vargas, Lanccon
         and Leitherer and respective collaborators) independently in
         Madrid, Strasbourg and Baltimore, offering theoretical
         results, covering the full wavelength range, as a function of
         the IMF and SFR, for a complete grid of cluster parameters
         (mass, age and metallicity). The active collaboration between
         the three PIs responsible for the codes permits us to test
         the models, and to be able to synthetize in great detail the
         whole UV-IR spectrum, as well as the ionizing photon spectrum
         and therefore the associated emission line spectrum of the
         gas. Star-forming regions, like the actual case of Starburst
         galaxies, require a special treatment in theoretical models
         to study their stellar content. Since the dominant population
         in these systems has to be a very young stellar cluster
         capable of ionizing the surrounding gas and producing the
         observed emission line spectrum, evolutionary synthesis
         models must include a photoionization code. The properties of
         the young clusters hidden inside the nebula are controlled by
         the most massive stars. These properties have to be derived
         from the emitting gas in an effort to un-mask the stellar
         population. For a given IMF, the evolution of massive stars
         (more than 15 solar masses) and their spectral energy
         distributions, as functions of the metallicity, will be the
         main inputs for the synthesis codes. The study of how the use
         of different evolutionary tracks and atmospheres can affect
         the output results is crucial, and for this reason Garcia
         Vargas and Leitherer (1995 in preparation) have tested their
         respective codes to control the effects of the evolutionary
         tracks and atmospheres models involved. The
         synthesis+photoinization part is better described by Garcia
         -Vargas's models. Leitherer's code takes into account the
         mechanical energy input due to massive stars. It also makes a
         very specific spectral synthesis to interpret the high
         resolution profiles of massive stars' lines in HST UV
         spectra, in terms of IMF parameters and cluster ages.
         Finally, the extension to the IR will not possible without
         Lanccon's models. Strong IR constraints like the calcium
         triplet observations (near-IR) and CO observations (at 2.3 microns
         will be use to test the presence of red supergiants or
         giants, whose evolutionary stages would correspond to very
         different populations in this galaxy. Therefore the
         disentangle between the existence of a previous burst rich en
         RSG, or the presence of a dominant old bulge population will
         be necessary to unveil the star formation history of this
         galaxy. From a preliminary analysis of the IR data, and
         according to evolutionary synthesis models, we find that CO
         is not deep enough to allow for the K band spectrum to be
         completely dominated by normal red supergiants. Possible
         explanations are the relatively low metallicity of the
         nucleus (0.4 x solar), a continuum contribution due to hot dust
         (L band emission would test this), or a continuum
         distribution from young stars. In fact, this last approach,
         including a young burst (3-5 Myr) plus an older episode of
         star formation (10-15 Myr), agrees with all the observational
         optical-ir constraints, in particular the Wolf-Rayet WR
         features and the emission line spectrum of the gas in the
         optical range (3-5 Myr), and the detection of the calcium
         triplet in the near-IR, revealing the presence of red
         supergiant RSG stars. This hyphothesis is more consistent
         with the observations than the continuous 20Myr long burst
         suggested by Bernlohr (1993). The HeI 2.06/Br-Gamma ratio of
         0.56 +/- 0.08 is consistent with either an IMF upper mass
         around 50 and an age of a few Myr, or with an IMF upper mass
         above 70 and an old burst. HST spectroscopy will be able to
         dissentangle the two possibilities, constraining the IMF
         parameters with the use of evolutionary models described in
         section 3 and the deblended ultraviolet CIV and SiIV lines.
         As we have showed in the descriptions of data from NGC 7714,
         the GHRS spectrum as well as the WFPC2 image are needed to
         detect directly the massive stars associated with the nuclear
         starburst, and therefore to complete our multiwavelength
         study focused on the star-formation processes in starburst
         galaxies.

   Real_Time_Justification:

         No supporting observations are planned. In addition to the UV
         spectroscopy and imaging we ask from HST, we have already
         observed NGC 7714 with several telescopes (4.2 WHT and 1.0
         JKT Observatorio Roque de los Muchachos, La Palma Spain; 3.5
         NTT, ESO Obs. Chile, and the 2.3 m Siding Spring Observatory
         Australia), including spectroscopy, and broad and narrow
         imaging covering the optical and IR ranges. We have very high
         quality spectroscopical data in the optical --- 3200-10000 AA
         --- (Gonzalez et al. 1995). Considering the longer
         IR wavelengths, since the previous observations in the
         literature (Puxley & Brandt 1994) were not sufficient for our
         multiwavelength study, we obtained our own IR data in two
         different campaigns: the first one included J,H,K and 2.3 microns
         CO images (0.5"/pix) as well as a K band spectrum,
         corresponding to the central 1.5 x 3 arcsec, and obtained
         with CASPIR, a cross-dispersed grism spectrograph on the 2.3m
         telescope of the Siding Spring Observatory (Australia). The
         second campaign (NTT at ESO, Chile) includes spectroscopic
         data around the HeI 2.06 microns and Br-Gamma 2.17 microns
         recombination lines, and part of the CO absorption band, as
         well as K imaging. Finally, the IUE low resolution spectrum
         (Kinney et al. 1993) has been used to calculate the exposure
         times since most of the UV continuum comes from the nuclear
         starburst. The HST archive images will be also included in
         our analysis.


Calibration_Justification:              

      Additional_Comments:


Fixed_Targets                           
  Target_Number:1
    Target_Name:NGC7714
    Alternate_Names:
    Description:GALAXY,STARBURST,NUCLEUS,KNOT,STAR FORMING REGION,SPIRAL
       Position:RA=23H 36M 14.11S +/- 0.15S, 
                DEC=+02D 09'18.1" +/- 1",  
                PLATE-ID=0515    
! Most common specification format is
! RA=0H 0M 0.00S +/- 0S,
! DEC=0D 0' 0.0" +/- 0",
! PLATE-ID=0000
        Equinox:J2000
        RV_or_Z:V=+2808
!          RA_PM:                        ! Units are seconds of time per year
!         Dec_PM:                        ! Units are seconds of arc per year
!          Epoch:1983.683
!Annual_Parallax:
           Flux: V=12.7 +/- 0.6, SURF(V)=18 +/- 2.0, 
                 F-CONT(1400)=30 +/- 10E-15, 
                 B-V=0.5 +/- 0.1, E(B-V)=0.22 +/- 0.05  
  
! This is a template for a single visit containing a single exposure
! Repeat exposure and visit blocks as needed

Visits                                  ! Section 6
      Visit_Number:1
Visit_Requirements:                     ! Section 7.1

! Uncomment or copy visit level special requirements needed
! Most of these requirements (including ORIENT) will limit scheduling

        ! PCS MODE [Fine | Gyro]
        ! GUIDing TOLerance <angle>
        ! DROP TO GYRO IF NECESSARY [NO REACQuisition]
        ! ORIENTation <angle1> TO <angle2>
        ! ORIENTation <angle1> TO <angle2> FROM <visit>
        ! ORIENTation <angle1> TO <angle2> FROM NOMINAL
        ! SAME ORIENTation AS <visit>
        ! CVZ
        ! PARallel
        ! SCHEDulability <percentage>
        ! AFTER <visit> [BY <time1> [TO <time2>]]
        ! AFTER <date>
        ! BEFORE <date>
        ! BETWEEN <date1> AND <date2>
        ! GROUP <visit-list> WITHIN <time>
        ! PERIOD <time> AND ZERO-PHASE <date>
        ! SEQ <visit-list> WITHIN <time>
        ! ON HOLD [FOR <visit-list>]

  On_Hold_Comments:
    Visit_Comments:

     Exposure_Number:1                   
         Target_Name:NGC7714
              Config:HRS
              Opmode:ACQ
            Aperture:2.0
          Sp_Element:MIRROR-N1
          Wavelength:
 Optional_Parameters:BRIGHT=RETURN
                     SEARCH-SIZE=3
Number_of_Iterations:1
   Time_Per_Exposure:90S                    
Special_Requirements:ONBOARD ACQUISITION FOR 2-3  
   
     Exposure_Number:2                  
         Target_Name:NGC7714
              Config:HRS
              Opmode:ACCUM
            Aperture:2.0
          Sp_Element:G140L
          Wavelength:1317
 Optional_Parameters:
Number_of_Iterations:1
   Time_Per_Exposure:1659.2s
Special_Requirements:

     Exposure_Number:3                   
         Target_Name:NGC7714
              Config:HRS
              Opmode:ACCUM
            Aperture:2.0
          Sp_Element:G140L
          Wavelength:1542
 Optional_Parameters:
Number_of_Iterations:1
   Time_Per_Exposure:4542.4s   
Special_Requirements:

        Visit_Number:2

     Exposure_Number:4               
         Target_Name:NGC7714
              Config:WFPC2
              Opmode:IMAGE
            Aperture:PC1                
          Sp_Element:F380W
          Wavelength:3964
 Optional_Parameters:CR-SPLIT=0.5
Number_of_Iterations:1
   Time_Per_Exposure:1000.0S
Special_Requirements:

     Exposure_Number:5                 ! Section 6.5
         Target_Name:NGC7714
              Config:WFPC2
              Opmode:IMAGE
            Aperture:PC1                
          Sp_Element:F380W
          Wavelength:3964
 Optional_Parameters:CR-SPLIT=0.5
Number_of_Iterations:1
   Time_Per_Exposure:800S
Special_Requirements:POS TARG 0.25,0.25

! Uncomment or copy exposure level special requirements needed

        ! INTeractive ACQuisition FOR <exp-list>
        !  ONBOARD ACQuisition FOR 1-3
        ! SAVE OFFSET <id>
        ! USE OFFSET <id>
        ! POSition TARGet <x-value>,<y-value>
        ! SAME POSition AS <exp>
        ! DO FOR TARGets <targ-list>    ! FGS only
        ! PARallel WITH <exp>
        ! GROUP-FGS <exp-list> WITHIN <time>
        ! SPATIAL SCAN <#> [<exp-list> | SINGLE-EXP ]
        ! RT ANALYSIS [FOR <exp-list>]
        ! REQuires UPLINK
        ! REQuires EPHEMeris CORRection <id>
        ! END ORBIT
        !  EXPAND
        ! LOW-SKY
        ! MAXimum DURation [<time> | <%>]
        ! MINimum DURation [<time> | <%>]
        ! NO SPLIT
        ! PHASE <number1[0,1]> TO <number2[0,1]>
        ! SEQuence <exp-list> NON-INTerruptible
        ! SHADOW

            Comments:

Data_Distribution                       ! Defaults indicated; change if desired

         Medium:        8MM             ! 8MM or 6250BPI or 1600BPI
Blocking_Factor:        10              ! 10 or 1
        Ship_To: PI_Address
       Ship_Via:        UPS             ! UPS (2-day) or OVERNIGHT
Recipient_Email:                        ! Needed if Ship_To: is not PI_Address
                                        ! <free-text>